The effects of N-(3',4'-dimethoxycinnamoyl)anthranilic acid (Tranilast), a blocker for histamine release from mast cells, on rabbit blood platelet functions were investigated. Tranilast inhibited dose dependently both the release of a lysozomal enzyme, beta-N-acetylglucosaminidase, and aggregation of washed rabbit platelets stimulated by thrombin and collagen. The IC50 was suspected to be 164 +/- 50 microM. The data suggest that Tranilast may be used as an anti-platelet agent in addition to an anti-allergic one.