The receptor-mediated binding of C1q to human phagocytic cells was investigated in this study. By using a C1q binding assay, we determined that purified, elutriated monocytes expressed an average of 4.6 X 10(5) C1q receptors (C1qR) per cell, with an equilibrium binding constant (Keq) of 0.91 X 10(7) (M-1). When analyzed in an identical manner, the polymorphonuclear leukocytes (PMN) expressed an average of 4.2 X 10(5) C1qR per cell, with a Keq for C1q of 1 X 10(7) (M-1). Fluorescent flow cytometric analysis showed that C1q was bound by 98% of the monocytes studied. Further, the pattern formed by these cells was consistent with a normal log distribution, indicating that this was a homogeneous population of cells. When PMN were assayed with flow cytometry, however, we found that C1q was bound by an average of only 45% of the PMN analyzed. Further, these PMN were not dispersed in a normal log distribution, indicating some heterogeneity among the cells that bind C1q. We examined the abilities of the chemoattractant N-formylmethionylleucylphenylalanine (fMLP) and the phorbol ester phorbol dibutyrate (PDBu) to modulate expression of C1qR as compared to the receptor for C3b (CR1). Pretreatment of the monocytes and the PMN with either 10(-6)M fMLP or 10 ng/ml of PDBu significantly increased cell surface CR1 expression, as reported previously by other investigators. In contrast, no significant increase in the number of C1qR on the monocytes or the PMN was observed with any of the concentrations of fMLP or PDBu used during pretreatment. However, with certain pretreatment doses of these agents, some reduction was noted in the amount of 125I-C1q bound to the monocytes and the PMN. This study characterizes the binding of C1q to purified monocytes and confirms previously published values for PMN. The distribution of cells expressing C1qR is shown to be significantly different between identically treated populations of monocytes and PMN. Finally, the abilities of fMLP and PDBu to modulate the binding of C1qR are examined. Our results indicate that the control of C1qR expression differs markedly from that of CR1.