Coumarin is known to exert an anti-neoplastic action in patients with malignant melanoma and renal cell carcinoma. We examined the effects of various concentrations (0, 10, 50, 100, 250, 500 micrograms/ml) of coumarin on the proliferation of two renal cell carcinoma cell lines (786-O and A-498) and two malignant prostatic cell lines (DU145 and LNCaP). After 5 days of treatment, coumarin inhibited the growth of the four cell lines. The LNCaP prostatic cell line was more sensitive than the other cell lines to the inhibitory effects of coumarin. We also examined the effects of various concentrations of suramin (0, 50, 200 and 400 micrograms/ml), and anti-neoplastic compound currently undergoing clinical trials, on the growth of the 786-O and LNCaP cell lines. Both of these were inhibited by suramin at concentrations greater than 100 micrograms/ml. Finally we examined the effects of combined treatment with coumarin and suramin on the growth of LNCaP and 786-O cells. When combined with all concentrations of suramin studied (0, 50, 200 and 400 micrograms/ml), coumarin further inhibited cellular proliferation of the LNCaP and 786-O cell lines. However, the combination of the highest concentration of coumarin (500 micrograms/ml) with suramin at 400 micrograms/ml did not inhibit the proliferation of the cells more than 500 micrograms/ml coumarin alone.