Panaxytriol, a polyacetylenic compound, isolated from red ginseng (Panax ginseng C.A. Meyer), was studied to determine its effects on the growth and cell cycle of tumor cell lines. The compound showed both significant cytotoxicity and inhibition of DNA syntheses in various tumor cells tested. For P388D1, a mouse lymphoma cell line, IC50 values for cytotoxicity and inhibition of DNA synthesis were 3.1 and 0.7 microg/ml, respectively. The cytotoxic effect of panaxytriol was both time- and dose-dependent. It also induced the cell cycle arrest of P388D1 at the G2/M phase, which was measured through flow cytometry. Particularly, the proportion of cells in the G2/M phase of the cell cycle increased from 9 % to 26 and 48 %, respectively, after 24 and 36 h exposure to panaxytriol at 5 microg/ml. There were corresponding decreases in the proportion of cells at the G0/G1 phase. The S phase also decreased during the 36-h treatment.