Tryptase, the dominant neutral protease of human pulmonary mast cell secretory granules, has the capacity in vitro to generate C3a anaphylatoxin from purified human C3. Only the alpha-chain of C3 is cleaved, and major fragments with apparent m.w. of 105,000, 39,500, 34,000, 29,000, and 9000 are detected by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis under reducing conditions. Fragments of 34,000 and 9000 m.w. are detected without reduction. A portion of the 9000 m.w. protein corresponds to C3a by virtue of its co-migration in SDS polyacrylamide gels with purified C3a and with trypsin-generated C3a, by its detection in a radioimmunoassay for C3a, and by its contractile activity on the guinea pig ileum bioassay. In the presence of heparin, another component of the mast cell secretory granule, the rate of appearance and the distribution of C3 cleavage fragments as assessed in SDS polyacrylamide gels are not appreciably changed with the exception that no C3a material can be detected in the SDS polyacrylamide gels or by radioimmunoassay and bioassay of the unresolved reaction mixture. Enhanced catabolism of authentic C3a by tryptase occurs in the presence of heparin and by analogy when C3a is generated from C3 by tryptase in the presence of heparin. Whereas tryptase secreted by activated human mast cells may generate C3a, a potentially important additional mediator of immediate hypersensitivity events, the concomitant release of heparin may serve to down-regulate C3a irrespective of its mechanism of generation.