We analyzed the sensitizing activity of five agents, which have been assumed to be MDR-overcoming drugs, in two human renal cell carcinoma cell lines expressing the MDR1 gene at high levels. In addition, we studied the sensitizing activity of cepharanthin, an MDR-overcoming agents, to vinblastine, adriamycin, epirubicin, cisplatin, mitomycin C and etoposide. Based on the results, we treated 6 patients with metastatic renal cell carcinomas (4: bones, 2: contralateral kidneys) by intraarterial injection of vinblastine and adriamycin (or epirubicin) in combination with cepharanthin. Two of the 4 bone metastasis cases responded markedly to the treatment. Renal tubular impairment was observed in one patient who was treated for the contralateral kidney metastasis.