Baltimore's Monumental Paper Co., best known for its modern-art-decorated delivery trucks, was sold yesterday to the nation's largest paper distributor, Valley Forge, Pa.-based Alco Standard Corp.

Soll Selko, Monumental's president, said last night that he and his family would remain in control of the 55-year-old packaging and chemical distribution company, which was founded by his father. He also said the company will retain its distinctive trucks, which are painted with red, blue and yellow squares in the style of artist Piet Mondrian.

Mr. Selko refused to disclose the terms of the sale, saying only that Alco had paid stock for the company and for Monumental's warehouse in Southwest Baltimore.

All though he had qualms at first when Alco asked if he would be interested in selling the company, Mr. Selko said that he was excited by the sale because Alco would fund Monumental's growth.

Mr. Selko said that he expected to add computer paper, fax paper and other products to his current line of packaging, tissue, shampoos and cleaning supplies.

Monumental, which expects to do about $22 million worth of business this year, hopes to double its revenues in five years, he said.

David Kirkland, a spokesman for Alco, said that the company bought Monumental in order to gain entry into the Baltimore-Washington market. He said that Alco has been buying up office products and distribution companies for years and is on the lookout for more acquisitions.