Stein, William Howard (1911-1980) was an American biochemist who helped reveal the complete chemical structure of the enzyme ribonuclease for the first time. He and his collaborator, chemist Stanford Moore, won the 1972 Nobel Prize in chemistry for their discoveries regarding the chemical structure of the enzyme ribonuclease and studies on deoxyribonuclease. They shared the prize with American scientist Christian Boehmer Anfinsen for his related achievements.

Stein graduated from Harvard University in 1933 and received his Ph.D. degree from Columbia University in 1938. In 1937, he joined chemist Max Bergmann's research group at the Rockefeller Institute (now Rockefeller University).

After World War II (1939–1945), Stein and Moore began to work together again to study the chemical makeup of proteins. In order to understand protein chemistry fully, it was necessary to be able to separate and analyze amino acids, or peptides, in large quantities, and to be able to purify proteins through chromatography, a separating and filtering process developed during the war, but there existed no easy way of doing so. They invented a drop-counting automatic fraction collector and spent the next several decades successfully researching and elucidating the precise chemical makeup of proteins.

They first focused on analyzing amino acids and later did groundbreaking work on the structural analysis of ribonuclease, an enzyme that promotes the hydrolysis of ribonucleic acid. Their invention became the prototype of similar commercial instruments, used for chromatography. Stein later adopted his method and, with Darrel H. Spackman, improved researchers' ability to study amino acids. Their discoveries have led to major advances in biochemistry and medicine.

Stein also received the Award in Chromatography and Electrophoresis in 1964 and the Richards Medal in 1972, both from the American Chemical Society, as well as the Kaj Linder-strom-Lang Award in 1972.