Passive cutaneous anaphylaxis (PCA) mediated in rats by IgE-like antibodies against egg albumin or the benzylpenicilloyl determinant was inhibited in a dose-dependent manner by intravenous treatment with Y-12,141; the ED50 was 0.09–0.2 mg/kg. The inhibitory effect of Y-12,141 on PCA was about 5 times as potent as that of disodium cromoglycate (DSCG). Oral treatment with Y-12,141 resulted in the inhibition of PCA, showing an ED50 of 2.5 mg/kg. This action of Y-12,141 on PCA was considered to be due to the inhibition of the release of allergic mediators from mast cells in a manner similar to DSCG. The present results suggest that Y-12,141 may have an anti-allergic activity.