Hollis Thomases' employees never shout across the office when they want to chat. Nor do they have to pick up the telephone to reach a handful of staffers who work long distances from WebAdvantage.net's headquarters in Aberdeen.

All they do is type out a quick line or two through Yahoo! Messenger.

"It's instantaneous communication," said Thomases, president of the firm, which handles online marketing for businesses. "And it's free, readily available to vendors and the people who hire us."

Although teen-agers and college students discovered instant messaging and made it a way of life, thousands of businesses across the country are now adopting the same America Online Instant Messenger, Microsoft Messenger, ICQ and Yahoo! Messenger programs that people use at home.

According to Robert Batchelder, a research director at Gartner Inc., a technology consulting firm, more than 50 percent of American businesses are using some form of IM.

Many companies are using consumer versions of IM, while others use "enterprise" releases developed specifically for businesses, with the free software building demand for the business-oriented programs.

"By 2006, instant messaging will surpass e-mail as the preferred method of business communication online," Batchelder said.

The IM concept isn't new. Businesses that rely on instant communications, including brokerages and newspapers, have had short messaging capabilities built into proprietary computer systems for 20 years or more. But as mainstream companies installed PC systems, they were more likely to rely on e-mail for long messages and the phone for instant communications. That is, until the Internet and mass instant messaging for consumers came along.

IM's great triumph is that it tells you before you send your message whether the recipient is online.

"IM is kind of like a typed phone call," Batchelder said. "Think of the 15-second meeting. As you know, often it takes about five minutes to decide something - but it might take three days to get everyone together who will make the decision, so it's three days and five minutes to decide something. IM greatly accelerates the speed at which companies can do things."

IM became a historic part of cyberspace in 1996, when four Israelis created a free program called ICQ (as in "I seek you"), which allowed people anywhere in the world to "talk" directly to one another over the Internet. Millions of users downloaded ICQ, and America Online bought out its parent company, Mirabilis, in 1998. Meanwhile, AOL expanded its own Instant Messenger program to allow non-subscribers to communicate with AOL customers and each other through AOL's servers.

Thomases said IM has few downsides in her business. In fact, she asks all her employees to sign onto IM when they get to work. "I can see when they're online, so IM allows for instantaneous communication," she said.

Some businesses shy away from IM, fearing that employees will waste time chatting with friends, but Thomases called that approach short-sighted.

"It's the same thing as when people take advantage of the smoke breaks or come back from lunch late all the time; that's a pattern of behavior," she said.

She added that e-mail remains useful, particularly for documents and longer messages that recipients may want to keep. It's also easier to use e-mail to send a single message to multiple recipients.

Josh Itzoe uses IM both socially and professionally as an account executive at Dataprise, a Rockville company that develops business networks. His main complaint is the lack of interoperability between IM programs and systems.

"If I use Yahoo! and someone uses MSN or someone uses AOL, I'm not able to communicate, and that can be cumbersome," he said. That requires his friends or clients to agree ahead of time on which system they'll use. His friends use Yahoo! Messenger; a former boss required him to get MSN Messenger.

"But I don't pay for instant messaging, so that's the upside," Itzoe said.

Mark Wesker, CEO of Artifact Software in Baltimore (www.artisansoftware.com), built a modified version of MSN Messenger into a program his company designed to help developers organize, track and reuse scraps of computer code they've written for various projects. The inclusion of IM allows the 6,000 users who have downloaded his Artifact Desktop program to ask questions of each other at any time.

"Developers hate telephones, but they love instant messaging," Wesker said. "I end up instant messaging people who are using my product at all times [of the day]. I'm doing this at 11:30 at night and getting marketing suggestions. Am I going to call someone at 11 o'clock at night? No, but I can ask them [through IM], 'Do you like the product?' And boom, they're telling me."

Others argue that IM can help businesses manage their computer resources better.

"Think about the e-mail where someone says, 'Ready to go to lunch,'" said Larry Letow, vice president of Provision Solutions, an IT consulting firm in Silver Spring. "No one ever deletes that type of e-mail."

An instant message, on the other hand, can simply disappear into the ether unless someone decides it's worth saving.

Consumer versions of instant messaging programs can cause problems, however. Less than 1 percent of businesses actually make an effort to manage the IM software installed on their PCs, Gartner's Batchelder said.

He and other Web security experts note attachments to instant messages often aren't filtered, which means viruses can slip in easily through a company's defenses.

The Social Security Administration, for example, considers IM programs inappropriate installations on its computers, said Mark Lassiter, a spokesman for the government office. Social Security has some 110,000 computer work stations nationwide. "Our biggest concern is that it makes us vulnerable to computer viruses, so we ask that employees not download those programs," Lassiter said.

Some companies' firewall software, designed to keep out intruders, won't allow attachments to come through, while other firewalls don't allow IM messages to flow through at all.

Another problem is that consumer IM technology doesn't encrypt messages - meaning sensitive information may be intercepted between the sender and receiver. Nor can most technology coordinators tie consumer IM systems into the authentication systems used to allow employees access to a business' computers.

More than 20 companies - IBM Lotus, Jabber, FaceTime and WiredRed among them - have created programs aimed at businesses to address many of the security issues, Batchelder said.

He said makers of the consumer IM versions also are getting into the enterprise arena, with Yahoo!, AOL, AT&T; and Sprint among others planning to offer business IM products.