The blood supply in the central portions of solid Yoshida sarcoma tumors is poor, but these portions contain viable cells capable of tumor growth when transplanted into new hosts. Attempts were made to increase penetration into these portions in rats by an injection of prostaglandin E1 together with lissamine green dye (used as a marker of drug penetration). The lissamine green dye defined the regions which could not readily be reached by blood-borne substances. Next, mitomycin C in place of lissamine green was administered together with prostaglandin E1. The growth of Yoshida sarcoma cells transplanted subcutaneously was suppressed significantly in the rats given mitomycin C with prostaglandin E1 compared with that in the rats given mitomycin C alone. The effect was greater when prostaglandin E1 was administered by an intraarterial route than by an intravenous route (P less than 0.01). Five patients with hepatic metastases from colorectal carcinoma were treated with a one-shot infusion of mitomycin C together with prostaglandin E1. Two patients showed objective evidence of improvement.