Four components were isolated from Cho-Rei (Polyporus umbellatus), one of the traditional Chinese medicines, and three of them potentiated the rabbit platelet aggregation response induced by collagen and/or adenosine-5'-diphosphate in vitro. The four components were identified as ergosta-7, 22-dien-3-one, ergosta-7, 22-dien-3-ol, 5α, 8α-epidioxy ergosta-6, 22-dien-3-ol and the already known component, ergosterol. When the C₃-hydroxy group of the active components was replaced by a ketone group, the enhancing activity disappeared, and an inhibitory tendency was observed. This suggests a relationship between bioactivity and the C₃-hydroxy group (or polarity) of these components.