Mac-1 is a macrophage surface antigen containing noncovalently associated alpha and beta subunits of Mr = 170,000 and 95,000, respectively (Kürzinger, K., and Springer, T.A. (1982) J. Biol. Chem. 257, 12412-12418). To determine whether the subunits are derived from a common or separate precursor, the biosynthesis of Mac-1 was studied. [35S]Methionine pulse-chase-labeled material was immunoprecipitated with either a monoclonal antibody recognizing an alpha chain determinant present in the associated alpha 1 beta 1 complex or a polyclonal antiserum recognizing the alpha 1 beta 1 complex as well as the free beta subunit. In peritoneal exudate macrophages, the alpha subunit was derived from a precursor of Mr = 161,000 which was converted to the mature Mr = 170,000 chain with a t1/2 of 30 to 45 min. The beta subunit was derived from a Mr = 87,000 precursor which became associated with the alpha subunit and was converted to Mr = 95,000 with a t1/2 of 2 h. Labeled beta chain took longer than alpha to become associated with the alpha 1 beta 1 complex in a number of different types of peritoneal macrophage populations, correlating with synthesis of an excess of beta. In the P388D1 macrophage-like tumor line, alpha and beta were processed with t1/2s of about 2 and 1 h. Both alpha and beta precursors were present in the complex, suggesting that complex formation preceded processing.