The complement peptides C3a and C5a have been shown previously to release histamine from human basophils but not human lung mast cells. As skin mast cells differ from those of the lung in both immunocytochemical and functional properties, we examined the ability of these anaphylatoxins to release preformed and newly generated mediators from human dispersed skin mast cells. In concentration-response studies, both C3a and C5a released histamine in a concentration related manner with C5a being 40-50 times more potent. However, the extent of histamine, 15-20%, was considerably less than that released from basophils. This was not due to catabolism of the peptides by mast cell proteases, mast cell supernatants that contained C5a being effective in releasing basophil histamine. Removal of the C-terminal arginine from C3a and C5a abolished their activity on skin mast cells. In time-course studies, histamine release induced by C3a and C5a was complete within 15 seconds. Complement-induced histamine release is a non-cytotoxic process as evidenced by 2-deoxy-D-glucose and antimycin A, inhibitors of glycolysis and oxidative phosphorylation, respectively. In contrast to IgE-dependent stimulation, anaphylatoxin-induced histamine release from human skin mast cells is independent of extracellular calcium. Both C3a and C5a at concentrations that induced 10-16% net histamine release caused a negligible release of the newly generated mediator, PGD2. The results suggest that C3a and C5a stimulate human skin mast cells in a manner similar to substance P and related basic secretagogues. However, the activation site for C3a and C5a appears to be different to that for substance P as the substance P antagonist (D-Pro4, D-Trp7,9,10) SP4-11 inhibited histamine release stimulated by substance P but not that induced by C3a and C5a.