Effect of the aqueous extract of Cinnamomum Cassia (CCAq) on experimental allergic reaction was investigated. IgE mediated reactions, homologous passive cutaneous anaphylaxis (PCA), degranulation of mast cells, and the release of histamine from sensitized lung tissues classified as the type I reaction by Coombs and Gell were not affected by CCAq. Complement dependent reactions including reversed cutaneous anaphylaxis (RCA), Forssman cutaneous vasculitis (FCV), and nephrotoxic serum (NTS) nephritis classified as type II and the Arthus reaction classified as type III were clearly inhibited by CCAq. However, CCAq did not affect the nephritis caused by the F(ab')2 portion of the nephrotoxic IgG antibody. CCAq in a high concentration inhibited the immunological hemolysis, chemotactic migration of neutrophils in response to complement activated serum, and the generation of chemotactic factors. The type IV reaction, contact dermatitis, was not affected by CCAq. The production of hemolytic plaque forming cells was slightly inhibited by CCAq. These results suggest that CCAq has an anticomplement action and inhibits the complement dependent allergic reaction.