The effect of different levels of dietary protein on 5-fluorouracil (FUra) toxicity and antitumor activity was assessed in female (BALB/c X DBA/2Ha)F1 (CDF1) and BALB/c mice bearing the P1798 lymphosarcoma. In the toxicity experiments, CDF1 mice were fed diets containing 5% [low protein (LP)], 20% [normal protein (NP)], or 60% [high protein (HP)] casein for 22 days and were then given a single ip injection of 275 mg FUra/kg. As determined by effects on the white blood cell count and by changes in body weight, FUra was least toxic in mice fed the HP diet. Moreover, mortality was 77.0, 42.9, and 11.4% for mice fed the LP, NP, and HP diets, respectively. In the antitumor studies, BALB/c mice bearing the P1798 lymphosarcoma were fed one of the 3 diets starting 5 days after transplant. Mice were given injections of 50 mg FUra/kg or the vehicle control on days 14 and 15 after transplant. As measured by tumor weights on day 16, FUra was least effective in mice fed the LP diet and most effective in mice fed the NP and HP diets. Survival time of mice given injections of FUra on days 14 and 15 was increased when compared to the survival time of control mice, but no difference in the percent increase in survival time was noted among the 3 dietary groups. If mice were given two cycles of FUra therapy (days 14 and 15 and days 20 and 21), mice fed the HP diet survived significantly longer than did mice fed the LP or NP diet. These data demonstrate that the feeding of an HP diet markedly decreases the toxicity of FUra while preserving or actually enhancing the antitumor effect.