We examined the effect of suplatast tosilate (IPD-1151T), which exhibits a class-specific suppression of IgE antibody production, on types I-IV allergic reactions. 1) Type I reaction: IPD-1151T dose-dependently inhibited 48-hr homologous passive cutaneous anaphylaxis (PCA) when the agent was given p.o. 30 min prior to the antigen challenge. To observe the time course of the inhibitory activity, IPD-1151T in a dose of 50 mg/kg was given orally at various times prior to the antigen challenge. IPD-1151T showed inhibitory activity when it was given at 0.5 to 2 hr prior to the antigen challenge, and the maximum inhibition was found when the IPD-1151T pretreatment was 2-hr before the challenge. IPD-1151T also suppressed the antigen-induced degranulation of mesenteric mast cells and histamine release from peritoneal exudate cells of rats. 2) Type II reaction: Only a high dose of IPD-1151T given orally inhibited reversed cutaneous anaphylaxis in rats, whereas the agent did not affect Forssman shock in guinea pigs. 3) Types III and IV reactions: IPD-1151T neither affected the Arthus reaction in rabbits nor the picryl chloride-induced contact dermatitis and sheep erythrocytes-induced footpad reaction in mice. The results obtained here indicate that IPD-1151T shows a relatively specific suppression of the type I allergic reaction with the inhibition of degranulation and histamine release from mast cells.