Membrane C1q (mC1q) of macrophages (MΦ) is a precursor of the IgG-binding serum protein C1q. Thus, mC1q potentially provides one of several Fcγ binding sites of mature MΦ and we analyzed whether simultaneous expression occurs of established receptors for IgG, FcγRI, II, and III, and mC1q during in vitro differentiation of MΦ. Using flow cytometry, immunoprecipitation combined with Western blotting and Northern blot analysis mC1q was hardly detected in freshly isolated blood monocytes, but increasingly in developing monocyte-derived MΦ. Laser scanning fluorescence microscopy confirmed the membrane localization of mC1q. Two-color-staining flow cytometry experiments indicated that mC1q and all three types of FcγRs are simultaneously expressed on mature monocyte-derived MΦ. A high correlation was found for the expression of mC1q and FcγRs, in particular FcγRII, but not mC1q and CD14, another marker of monocytes/MΦ.