We revealed an antimetastatic effect of thymus-dependent corpuscular antigen (sheep erythrocytes) injected intravenously or intraperitoneally in sensitizing or high doses alone or in a complex with a course dose of cyclophosphamide to C57Bl/6 mice with Lewis carcinoma. Injection of the antigen appreciably reduced the number and volume of Lewis carcinoma metastases in the lungs, notably increased the therapeutic effect of cyclophosphamide, and restored hemopoiesis, particularly, the red blood stem suppressed by the tumor process and cytostatic treatment. The growth of primary tumors virtually did not change. High dose of sheep erythrocytes was more effective.