The antitumor effect of dicyclohexylammonium sulfate (DCHA), a potent inhibitor of spermidine synthase, was tested on BDF1 mice inoculated i.p. with P388 leukemia (1 X 10(6) cells/mouse). DCHA prolonged the survival time of mice bearing P388 leukemia at the doses of 10-100 mg/kg administered daily for 6 days. The spleen weight increased by 30% at 7 days after tumor inoculation. DCHA treatment had no effect on the tumor-induced increase in splenic weight. The spermidine concentration of the ascites tumor cells and spleens of mice bearing the tumor was lowered by the treatment, while spermine concentration hardly changed. The depletion of spermidine in the ascites tumor cells and spleens might be a cause of the suppression of tumor growth.