Shares of Digene Corp. and Cytyc Corp. plummeted yesterday, one day after the Federal Trade Commission said it would try to block the companies' planned merger.

Both companies hit 52-week lows in trading yesterday. Digene, headquartered in Gaithersburg, shed $2.73 a share, or 21.5 percent, to close at $9.96, and Boxborough, Mass.-based Cytyc plunging 40.7 percent, or $4.66, to close at $6.80.

The companies said they were reviewing their options in the wake of the FTC announcement, which was made after the close of trading Monday.

Digene said it "continues to support the proposed transaction."

Analysts said it is unlikely that the $420 million acquisition can proceed. "You have to assume, in the face of FTC opposition, that they can't keep this thing together," said Tom Burnett, president of Merger Insight, the research affiliate of Wall Street Access, a brokerage firm.

The FTC said the deal -- which would link Cytyk, the leading maker of Pap tests for cervical cancer, with Digene, which makes a test to confirm Pap results -- would lead to decreased competition, limits on innovation and higher consumer costs for cervical cancer screenings. The commission voted 5-0 to seek a court injunction blocking the sale as anti-competitive.

"Most people thought it was a logical marriage," said Linda Miller, who manages the $400 million John Hancock Health Sciences Fund, which holds Cytyc shares. "You had one company with the test and one with the collection and analysis device. It seemed to be a good fit. I guess it was too good a fit."

The acquisition "is history or close to it at this point," Miller said.

The FTC noted that Digene makes the only approved DNA-based test for a virus that causes nearly all cervical cancers. "By purchasing Digene, Cytyc would be in a position to eliminate its only existing competitor [TriPath] by limited access" to Digene's test, the FTC said.

The FTC's decision "limits our options to either litigation against the FTC or potentially abandoning the transaction," Patrick Sullivan, chief executive officer of Cytyc, said during a conference call.

He said the company was negotiating with the FTC but wouldn't comment on the discussions. "We strongly believe that the commission has reached the wrong outcome," Sullivan said.

Burnett said companies can sometimes solve antitrust problems by selling assets but that "this deal doesn't lend itself to divestiture because Digene is basically a one-product company."

Bloomberg News contributed to this article.