The collagen-like C1q molecule, a subcomponent of the first component of complement, C1, is synthesized by macrophages (M phi). Previously, we have demonstrated that C1q is a membrane protein of guinea pig peritoneal macrophages (M phi). To extend this observation as a general biologic characteristic of M phi, we investigated human (hu) monocyte-derived M phi. Interestingly, surface labeling with the biotin derivative sulfosuccinimidyl-6-(biotinamido)-hexanoate of M phi, freshly isolated monocytes, lymphocytes, granulocytes, and myelomonocytic U937 cells revealed that C1q occurs only on the surface of M phi and not on the surface of the other cells types. Therefore, C1q appears to be a marker for differentiation into M phi. FITC-labeled, fixed Staphylococcus aureus coupled to membrane C1q via a monoclonal alpha-hu-C1q Ab were used to demonstrate that membrane C1q is capable of mediating phagocytosis. Various detergents (Nonidet P-40, digitonin, lubrol, and Triton X-114) were used to solubilize membrane C1q. Membrane C1q of hu M phi is tightly bound to or located in the intact membrane, since treatment of cells with acidic buffers ("acid strip") failed to remove C1q from the cell surface. However, repeated freezing and thawing of cells and washing of segregated membranes with buffer containing 1 M KCl and 3 M urea brought about a marked release of membrane C1q.