The present experiment was performed to examine the effects of oxatomide on the four types of allergic reactions classified by Cooms and Gell. 1) type 1: Oxatomide administered p.o. showed inhibitory effects on the 48 hr homologous passive cutaneous anaphylaxis in rats and the passive anaphylactic bronchoconstriction in guinea pigs. 2) type 2: Oxatomide showed few effects on the complement-dependent cytolysis of sheep red blood cells, the Forssman shock in guinea pigs, and the reversed cutaneous anaphylaxis and the swelling of the footpad induced by rabbit antiserum against rat serum in rats. 3) type 3: Oxatomide alleviated the symptoms during the early stage of the active Arthus reaction in guinea pigs, but the drug did not inhibit the symptoms during the later stage of the reaction. 4) type 4: Oxatomide did not exert an inhibitory effect on the delayed-type hypersensitivity responses to picryl chloride and to sheep red blood cells in mice. These results indicate that oxatomide selectively suppress the type 1 allergic reaction.