Antitumor resistance decreased in mice 24 h after injection of cyclophosphamide in a dose of 100 mg/kg. This was seen from more rapid growth of Ehrlich's ascitic carcinoma transplanted intraperitoneally 24 h after cyclophosphamide injection and 17% reduction of the lifespan of mice with tumors. Three, 7, 10, 14, and 22 days after cyclophosphamide injection, the antitumor resistance increased and the lifespan of animals with Ehrlich's ascitic carcinoma transplanted at the corresponding periods increased by 20, 14, 42, 29, and 36%, respectively, in comparison with mice with transplanted tumor not injected with the drug. Injection of cyclophosphamide 1 day after tumor transplantation prolonged of the lifespan of animals with tumors by 76%.