Controversial results have been published in the past regarding the functional reactivity of different cell types to the anaphylatoxin C3a and its degradation product C3a(desArg). To understand better the effects of C3a and C3a(desArg) on human mast cells, the authors performed binding experiments and calcium mobilization studies on the human mast cell line HMC-1 which has been shown previously to express C3a binding sites. For this purpose, functionally active, recombinant C3a (rC3a) was constructed with an 11 amino acid peptide attached to the N-terminus of the molecule. Using a monoclonal antibody (MoAb) against this tag, binding of rC3a to HMC-1 cells could be demonstrated by flow cytometry. Its binding was specific as it could be blocked with serum-derived C3a. In contrast, no binding of rC3a(desArg) to HMC-1 cells was detectable. Recombinant C3a led to a transient mobilization of intracellular calcium [Ca2+]i in HMC-1 which was inhibitable by the C3a-specific MoAb K13/16. No increase of [Ca2+]i was detected when the cells were treated with C3a(desArg). The authors found C3a receptor (C3aR)-specific mRNA in HMC-1 cells indicating that this receptor represents the binding site for C3a on these cells. These results demonstrate a specific binding for C3a but not for C3a(desArg) on cells of the human mast cell line HMC-1. As a consequence, functional activity was restricted to C3a with C3a(desArg) being completely inactive. Therefore, the data strongly suggest that the recently cloned high affinity C3aR which is assumed to represent the binding site for the anaphylatoxin on HMC-1 cells is unresponsive to C3a(desArg).