Macrophage adhesion molecule (MAM), an abundant surface molecule which functions in the adhesion and spreading of guinea pig macrophages on surfaces, is characterized as a heterodimer of the trypsin- and plasmin-sensitive glycopeptide gp160 (MAM-alpha) and the glycopeptide gp93 (MAM-beta). The density of MAM molecules is estimated at 630,000 per macrophage on the basis of quantitative binding of 125I-labeled monoclonal antibody. The glycopeptide subunits display microheterogeneity on isoelectrofocusing; the pI is 5.8-6.3 for gp160 (MAM-alpha) and 6.4-7.0 for gp93 (MAM-beta). A neutrophil gp160, gp93 molecule was shown to be indistinguishable from macrophage MAM on the basis of electrophoresis, isoelectrofocusing, and reactivity with 10 monoclonal antibodies. A related heterodimer of gp93 associated with a larger, antigenically different glycopeptide (gp180,gp93) was identified on circulating lymphocytes. Cumulative properties indicate that MAM is the guinea pig analogue of human Mo1 and mouse Mac-1.