The effect of calceolarioside A, a phenylpropanoid glycoside (PhG), isolated from Calceolaria hypericina, was studied on rabbit platelets in vitro. Calceolarioside A induced a dose-related aggregant effect on rabbit platelets. Indomethacin did not modify the calceolarioside A-induced aggregant effect. Furthermore, no modification was exerted by phenoxybenzamine, BW 577C, and WEB 2086 on the PhG aggregant effect. On the contrary, TMB-8, an intracellular calcium blocker, significantly reduced the PhG effect. The latter result suggests that calceolarioside A aggregation may be partly related to a calcium-dependent mechanism.