A sensitive and specific fluorescence assay for membrane complement (C) receptors (CR1 and CR2) was developed with purified C3b and C3d fragments coupled to fluorescent microspheres (0.9 mu diameter). C3-microspheres (C3-ms) bound to cells with low numbers of receptors that were undetectable by other assay techniques. Inhibition studies with anti-CR1 and anti-CR2 demonstrated that C3b-ms and C3d-ms bound exclusively to CR1 and CR2, respectively. Preparation of the C3-ms required only small amounts of partially purified C3 and no immunoglobulin or other C components. Once formed, the C3-ms were stable for up to 4 mo at 4 degrees C.