We have shown previously that the flavonoid quercetin (3,3',4',5,7-pentahydroxyflavone) enhances the antiproliferative activity of cis-diamminedichloroplatinum(II) (cis-DDP) in vitro. In order to investigate whether this observation could be exploited in cancer treatment, we tested this drug combination in human tumor xenografts. The established human large-cell cancer of the lung (LXFL 529) was implanted s.c. into nude mice. Tumors were allowed to grow to a mean diameter of approximately 5 mm and the animals were subsequently treated intraperitoneally with quercetin, cis-DDP or a combination of both. Treatment was given 3 times at 3-day intervals. Twenty milligrams quercetin per kg body weight caused no inhibition in tumor growth compared to untreated controls; 3 mg cis-DDP per kg body weight with the same time schedule reduced tumor growth, compared to quercetin-treated and control animals. Concomitant treatment with 20 mg quercetin and 3 mg cis-DDP per kg body weight reduced tumor growth to a significantly greater degree than cis-DDP alone. Toxicity of this treatment was relatively low as determined by measurements of the body weight of the mice. A combination of 4 mg or 5 mg cis-DDP with 20 mg quercetin per kg body weight also reduced tumor growth compared to single cis-DDP treatment. The toxicity of treatment with these increased doses was high, as shown by the high lethality and the loss of body weight of surviving animals.