We investigated the effects of the powders of Spirulina platensis (SPP) on anaphylactic reactions. SPP inhibited compound 48/80-induced anaphylactic shock 100% with doses of 0.5, and 1.0 mg/g body weight (BW). SPP significantly inhibited serum histamine levels induced by compound 48/80 in rats. SPP (0.5 mg/g BW) inhibited to 68.7% passive cutaneous anaphylaxis activated by anti-dinitrophenyl (DNP) IgE. SPP dose-dependently inhibited the histamine release from the rat peritoneal mast cells (RPMC) by compound 48/80. Moreover, SPP had a significant effect on anti-DNP IgE-induced histamine release or tumor necrosis factor-alpha production from RPMC. These results suggest that SPP may contain compounds with actions that inhibit mast cell degranulation in the rat.