AS A WASHINGTON lawyer, Huntington Cairns kept better company than most. Plato, he once said, had dominated the first part of his life and Shakespeare the second. While the firm of Plato, Shakespeare and Cairns never commanded $250 an hour, it did leave its mark on the cultural life of this city and the country.

Mr. Cairns, who died last Saturday in Kitty Hawk, N.C., at the age of 80, had been away from Washington for two decades. He retired in 1965 as secretary, treasurer and general counsel of the National Gallery of Art, which he had served almost from its beginning.

During a successful law career, he had written and edited books on a number of diverse subjects, including his fellow Baltimorean, H. L. Mencken (who had advised him to go directly from high school to law scool -- advice he followed), jurisprudence, poetry, prose, anthropology and, of course, art. His best-known book perhaps was "The Limits of Art," an anthology of selections from the classics with commentaries by authorities.

He had a knack for popularizing, shown in such works as the "Collected Dialogues of Plato," which he edited with scholar Edith Hamilton. "I saw Edith Hamilton nearly every Sunday night for 20 years," Mr. Cairns once said. "One day I said, 'Let's do for Plato what the West has done for Shakespeare and the Bible.' She said, 'What's that?' I replied, 'Put him all in one volume on thin paper so you can put it in your hip pocket when you go to New York.' Mark Van Doren, he was one of the originators of the radio program "An Invitation to Learning."

Press attention to him centered on an unpaid sideline: his role advising the Treasury Department on works that should be banned from importation because they were pornographic. He wasn't a censorious sort and, in fact, had been asked to advise the government after he successfully defended a Baltimore book shop that was importing a translation of "Daphnis and Chloe" that some official thought was pornographic. Said Mr. Cairns of his sideline: "Someone had to do it. Most of the customs people didn't know a Vatican mural from a French postcard."

Mr. Cairns was, as one writer noted, an "inveterate student," but one whose edge seems never to have been dulled. When he turned 75, Paul Mellon, the National Gallery's chief benefactor, observed of him: "His encyclopedic knowledge and wide range of intellectual interests are astonishing. . . . A philosopher and a higher mathematician, he has always appeared to be living in a child's aura of delight and wonder."