The cytotoxic effect of four tanshinones isolated from the dried root of Salvia miltiorrhiza Bunge (Lamiaceae) were studied in vitro using P388 lymphocytic leukemia cells. Tanshinone I and tanshinone IIA were shown to be quite strongly cytotoxic against the cells (86.76% and 56.05% cell inhibition at 25 microg/ml). Dihydrotanshinone I and cryptotanshinone, which lack of saturation at C-15, showed little cytotoxicity (13.71% and 39.21% cell inhibition at 25 microg/ml). The results also revealed a structure activity relationship. We suggest that the unsaturation at C-15 and saturation on ring A may be critical structural components for cytotoxicity in these diterpenes.