The influence of cycloferon and its combinations with cyclo-phosphamide on the growth of transplantable tumors was studied in rats with lymphosarcoma of Pliss and CBA and NMRI mice bearing Ehrlich's carcinoma and leukemia L1210. While cycloferon alone failed to significantly influence tumor growth, treatment with 100 mg/kg of the drug was followed by longer survival in mice with transplantable Ehrlich tumor. The highest inhibitory effect of combined cycloferon and cyclophosphamide was recorded on day 10 when tumor growth inhibition was as high as 90%. Mitotic activity of tumor was inhibited following multiple treatment with cycloferon, in combination with cyclophosphamide and cyclophosphamide alone. Survival longer than in control was registered only in animals with L1210 receiving cyclophosphamide alone or in combination with cycloferon. Cycloferon administration was shown to inhibit the growth of Pliss lymphosarcoma only within a short period after the start of the study. Combined treatment with cycloferon and cyclophosphamide produced the highest inhibition. The antitumor effect of cycloferon varied depending on tumor pattern, species and line of animals and dose and modality of administration.