Laurence H. Silberman, former undersecretary of Labor and former ambassador to Yugoslavia, is resigning as executive vice president of San Francisco's Crocker National Bank to return to Washington as managing partner of his old law firm, Morrison & Foerster.

After three years as a banker, "I got to the position where I really wanted to be a lawyer and get back to practice," Silberman said. He will return to the Morrison & Foerster Washington office he opened when he joined the firm in 1978.

He said that he will specialize in banking matters and international and trade law. It is unlikely that he will sever all ties with Crocker National Bank, because it is among his firm's clients. Morrison & Foerster has its main office in San Francisco and offices in Los Angeles, Denver and London, as well as Washington.

Silberman served as solicitor in the Labor Department from 1969 to 1970, during the Nixon administration, and was promoted in 1970 to the job of Labor undersecretary. He resigned in 1972 to practice law with the Washington firm of Steptoe & Johnson, but in 1974 returned as deputy attorney general for the last six months of the Nixon administration and the first year of Gerald Ford's presidency. Ford named Silberman ambassador to Yugoslavia in 1975, where he served for two years. After that, he became a senior fellow at the American Enterprise Institute, writing and lecturing.