Antigenicity of suplatast tosilate (IPD-1151T) was investigated in guinea pigs and mice. The results were as follows: 1. Homologous passive cutaneous anaphylaxis (PCA), active systemic anaphylaxis (ASA), active cutaneous anaphylaxis (ACA) and Schultz-Dale reaction tests were carried out using guinea pigs which were immunized orally with IPD-1151T alone or subcutaneously with IPD-1151T and Freund's complete adjuvants (CFA). Positive reactions in these tests were not produced by eliciting injection of IPD-1151T or its metabolite, M-1. On the other hand, the sensitization of ovalbumin (OVA) with CFA produced positive reactions in all of PCA, ASA, ACA and Schultz-Dale tests. 2. Heterologous passive cutaneous anaphylaxis (PCA) test using rats was carried out using two strains of mice (C3H / He and BALB / c) which were immunized orally with IPD-1151T alone or intraperitoneally with IPD-1151T and aluminum hydroxide gel (Alum) as an adjuvant. No animals showed positive reaction to both eliciting antigens, IPD-1151T and M-1. On the other hand, a positive reaction in PCA test to eliciting antigen, OVA, was obtained in rats treated with sera of mice sensitized with OVA plus Alum. 3. These findings showed that IPD-1151T had no antigenicity in guinea pigs and mice.