A study was made of the content of alpha-fetoprotein (alpha-FP) in the blood serum and of proliferative processes in hepatoma 22A-bearing mice after injection of cyclophosphamide in a dose of 150 mg/kg at varying time of 4-day tumor growth. The data obtained indicate that the time of appearance of alpha-FP in the blood, the pattern of rhythmic fluctuations in protein content and in the number of DNA-synthesizing and mitotic cells of the tumor depend on the time of the cytostatic injection during the day. Besides, cyclophosphamide exerts a more powerful inhibitory effect on the content of alpha-FP in the blood serum and less powerful on the proliferative processes in hepatoma 22A.