Antitumor activity of bleomycin against Ehrlich ascites carcinoma in mice was examined in the presence and absence of a polyene antibiotic, pimaricin. A single intraperitoneal (ip) injection of bleomycin and pimaricin in combination into tumor-bearing mice on day 1 produced a synergistic life-prolongation effect as compared with that of each agent alone. Accumulation of abdominal ascites was almost completely blocked by bleomycin in combination with the tetraene antibiotic. Frequent ip injection of small doses of bleomycin and pimaricin on day 1 produced a much greater increase in life span than the single ip injection. To obtain effective synergism under these treatment schedules, it appeared that the dose of bleomycin should be reciprocally proportional to the dose of pimaricin.