Corosolic acid isolated from the fruit of Cratoegus pinnatifida var. psilosa was tested for anticancer activity. Corosolic acid displayed about the same potent cytotoxic activity as ursolic acid against several human cancer cell lines. In addition, the compound displayed antagonistic activity against the phorbol ester-induced morphological modification of K-562 leukemic cells, indicating the suppression of protein kinase C (PKC) activity by the cytotoxic compound. The compound showed PKC inhibition with dose-dependent pattern in an in vitro PKC assay.