Saikosaponin (SS) b2 was found to inhibit the proliferation of B16 melanoma cells. To explore this mechanism, we employed flow cytometry to determine the distribution of DNA content. The cell cycle of B16 melanoma cells was accumulated in the G1 phase followed by induction of apoptosis. This suggests that SSb2-induced proliferation inhibition is caused by G1 phase accumulation and that apoptosis induction is G1-phase-accumulation dependent. Phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, did not interfere with the proliferation and did not induce apoptosis of B16 melanoma cells by itself. However, PMA significantly abolished these effects of SSb2 in including proliferation inhibition and apoptosis induction. Down-regulation of the PKC activity may be involved in the effect of SSb2.