Ethylcellulose microcapsules of mitomycin C were prepared for intra-arterial use in an attempt to enhance the antineoplastic effects of mechanical arterial embolization. The potential therapeutic effects of intra-arterial mitomycin C microcapsules were considered to be a function of microembolization and prolonged drug action, that is chemoembolization. A total of 33 patients with renal cell carcinoma was subjected to transcatheter arterial chemoembolization with mitomycin C microcapsules supplemented with gelatin sponge, and 10 patients were subjected to chemoembolization with non-encapsulated mitomycin C and gelatin sponge. Comparative studies on objective tumor decrease, angiographical findings and histological effects between the 2 treated groups indicated that mitomycin C microcapsules definitely enhanced the therapeutic responses to gelatin sponge embolization. Bioassay of the peripheral blood revealed that intra-arterial mitomycin C microcapsules reduced markedly mitomycin C levels in the blood compared to non-encapsulated mitomycin C, leading to a decrease in hematological toxicity. The results may prove that chemoembolization with mitomycin C microcapsules is effective as a preoperative and palliative measure in the treatment of renal cell carcinoma.