The expression of specific membrane receptors for C3a was determined on guinea pig C3a-sensitive (gp R+) platelets and human polymorphonuclear leukocytes (hu PMNL). Binding studies with 125I-labeled C3a from gp or hu sources and Scatchard analysis applied to the binding data revealed the existence of two receptor classes on gp R+ platelets; a high-affinity class with about 200 binding sites/cell and Kd = 1.7 x 10(-9) M, and a relatively low-affinity class with Kd = 10(-8) M and about 500 sites/cell. Hu PMNL express a homogeneous receptor class with Kd = 3 x 10(-8) M and 40,000 sites/cell. Molecular characterization of the C3a receptor on gp R+ platelets was achieved by (a) cross-linking photoaffinity-labeled receptors to bound 125I-labeled C3a; (b) photoaffinity labeling receptors with a 13-amino acid residue C3a analogue 125I-Nap-Ahx-13; and (c) use of chemical cross-linkers like disuccinimidylsuberate to cross-link receptors with 125I-C3a. All three techniques gave rise to very similar labeling patterns. With the photoaffinity labeling methods, a diffuse band pattern was observed with an apparent molecular mass of 95-123 kDa with 125I-C3a as label, and 85-105 kDa with 125I-Nap-Ahx-13 as label. Chemical cross-linking of 125I-C3a revealed three distinct bands with molecular masses of approximately 123, 108 and 95 kDa. Subtracting the contribution of the cross-linked ligands, the C3a receptor on gp R+ platelets appears to be a protein complex, consisting of one to three components with estimated molecular masses between 83-114 kDa.