We have defined the expression of the macrophages (m phi)-specific antigen (Ag) F4/80 during differentiation in culture. The progenitor cells-the colony-forming unit in culture and cluster-forming cell-lacked Ag F4/80 but gave rise to colonies of F4/80-positive adherent m phi, as shown by fluorescence-activated cell sorting and clonal assays with L cell-conditioned medium as the source of growth factor. Ag F4/80 first appeared on a nonadherent precursor found in mass liquid BM cultures after 3 d. Once adherent, m phi expressed high levels of Ag F4/80 and other markers. The role of L cell-conditioned medium and of adherence on expression of Ag F4/80 was also examined. Clonal analysis of F4/80 and other Ag, Mac-1, and 2.4G2 (FcR) showed that all cells in all independent colonies come to express these markers. These studies establish that F4/80 is a marker for the more mature stages of m phi development and that Ag expression increases progressively during maturation in vitro. Heterogeneity of Ag expression can be ascribed to variation in development and not to independent subsets of the m phi.