In a search for antitumor agents, we carried out a screening of 4-arylcoumarins isolated from endophytic Streptomyces aureofaciens CMUAc130, by examining their possible inhibitory effect on the growth of s.c. transplanted Lewis lung carcinoma (LLC) in BDF-1 mice by intraperitoneal (i.p.) administration. The 4-arylcoumarins showed antitumor activity with T/C values of 80.8 and 50.0% at doses of 1 and 10 mg/kg of 5,7-dimethoxy-4-p- methoxylphenylcoumarin treatment, respectively and 81.5 and 44.9% at doses of 1 and 10 mg/kg of 5,7-dimethoxy-4-phenylcoumarin treatment, respectively, compared to adriamycin, which was used a positive control, with T/C value of 55.9% at 2 mg/kg. Furthermore, we investigated the possible effects of these compounds on expression of the bcl-2 and Bax oncoproteins in A427, a human lung cancer cell lines. The cells were cultured in vitro for 24h in RPMI 1640 with 1.5% (v/v) ethanol, 100 μg/ml 5,7-dimethoxy-4-p-methoxylphenylcoumarin or 5,7-dimethoxy-4-phenylcoumarin. Viability was determined by an MTT assay. Total protein was extracted from cell lysates and the bcl-2 and Bax oncoproteins were identified. Western blotting showed a decrease in bcl-2 and an increase in Bax in A427 cell cultured with 5,7-dimethoxy-4-p-methoxylphenylcoumarin or 5,7-dimethoxy-4-phenylcoumarin. We conclude that 5,7-dimethoxy-4-phenylcoumarin is a more potent inhibitor of cell proliferation than 5,7-dimethoxy-4-p- methoxylphenylcoumarin and has more marked effects on oncoprotein expression.