Recently, we have found for the first time that dihydrotanshinone I, isolated from Salvia miltiorrhiza, exhibited cytotoxicity against various tumor cell lines. To investigate whether the mechanism underlying dihydrotanshinone I toxicity involved apoptosis in cancer cell Lines, we examined cell growth arrest and cell death by flow cytometric analysis and DNA fragmentation assay. Dihydrotanshinone I induced cell growth arrest during the S phase and subsequently, apoptosis, following its application to K562/ADR cells, whereas cryptotanshinone did not have these effects. These results suggest that the mode of action of dihydrotanshinone I involves apoptotic pathways that are different from those involved in cryptotanshinone toxicity.