Earlier studies showed that approximately 26% of the cells present in human mononuclear cell preparations had the ability to bind purified monomeric C1q. The present studies were initiated to identify the cell types comprising the C1q binding population. Double marker fluorescence, rosetting, and morphologic studies on cell preparations depleted of or enriched in various cell types were simultaneously employed to identify those subpopulations that bound C1q. C1q binding was detected by fluorescent techniques (with FI-F(ab')2 anti-C1q). Monocytes in mononuclear cell preparations were detected by the ability to phagocytose carbonyl iron. B cells were identified by reactivity with rhodamine-conjugated F(ab')2 anti-human F(ab')2 and by rosetting with erythrocytes bearing C3b. These studies showed that monocytes and B lymphocytes comprised the majority of C1q-binding cells in mononuclear cell preparations, whereas T lymphocytes lacked this property. In addition, a minor population of nonphagocytic cells in such preparations that lacked B and T cell markers also bound C1q. Finally, a high but variable proportion of polymorphonuclear leukocytes bound C1q. Binding of C1q to PMN was concentration-dependent, saturable and specific and exhibited an equilibrium constant of 0.76 X 10(7) M-1. Thus, PMN also possess a specific receptor for C1q.