The complement anaphylatoxin peptide C5a is well known to activate human polymorphonuclear leukocytes through receptor-mediated processes. C5a has also been reported to activate eosinophils for both chemotaxis and hexose uptake. We characterized the receptor molecule for human C5a on human eosinophils and compared it with the receptor on human neutrophils. At 4 °C, uptake of 1 nM ¹²⁵I-C5a reaches equilibrium within 10 min on both cell types. Binding of ¹²⁵I-C5a occurs over a concentration range comparable to that which stimulates lysosomal enzyme release and hexose uptake in both cell types. Scatchard analyses of the data indicate the presence of two receptor populations on eosinophils; a high affinity receptor with 15,000–20,000 sites/cell and a K^d of 3.1 ± 0.6 × 10⁻¹¹ M, and a low affinity receptor with approximately 375,000 sites/cell and a K_d of 1 × 10⁻⁷ M. Parallel experiments with neutrophils indicate the presence of a single receptor population with approximately 90,000 sites/cell and a K_d of 4.8 ± 0.1 × 10⁻¹⁰ M. The eosinophil receptor molecule was further characterized by covalently cross-linking ¹²⁵I-C5a to cells followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the solubilized material. Autoradiography indicates the presence of a dominant C5a-eosinophil receptor complex with an apparent mass of 60–65 kDa. The corresponding neutrophil-C5a receptor complex has an apparent mass of 50–52 kDa as observed by others. When the cross-linked ¹²⁵I-C5a-receptor complex was treated with cyanogen bromide, different patterns were observed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis for neutrophils and eosinophils. Thus, human eosinophils have a receptor for C5a anaphylatoxin which appears to be distinct from the C5a receptor present on human neutrophils.