Synthetic antioxidant potassium phenosan in ultralow doses administrated in combination with antitumor antibiotic adriamycin in a therapeutic dose (8 mg/kg) markedly prolonged the mean life span of tumor-bearing animals compared to adriamycin monotherapy. This effect depended on the dose of antioxidant and was maximum at phenosan concentrations of 10(-17) and 10(-15) M. Potassium phenosan in these concentrations not only increased the mean life span, but also determined survival of 10-20% animals (as differentiated from adriamycin monotherapy).