We studied the effects of the powders of Spirulina platensis (SPP) on anaphylactic reactions. SPP inhibited systemic anaphylaxis induced by compound 48/80 in rats. SPP also inhibited local anaphylaxis activated by anti-dinitrophenyl(DNP) IgE. Moreover, SPP dose-dependently inhibited histamine release in rat peritoneal mast cells activated by compound 48/80 or anti DNP-IgE. These results suggest that SPP may contain compounds with actions that inhibit mast cell degranulation in the rat.