Fifteen to 16 cell surface proteins which interact with soluble components of the complement system have now been identified. Most of these--CR1, CR2, CR3, "CR4", DAF, HSV-1 c glycoprotein, Gp 45-70, p150,95, cell-surface Factor H, and a 90 kD protein--interact with C3 or C4 and their degradation products. Other receptors for C1q, Factor H, C5a, and the C5b-9 complex have been identified. Receptors for additional complement proteins such as Factor B or its fragments are likely to exist. Complement receptors have a wide tissue distribution and have major roles in controlling the turnover of the complement system, and regulating behaviour and growth of leukocytes.