We evaluated the effects of WR-2721 on the toxicity and antitumor activity of the combination of 5-fluorouracil (5FU) and carboplatin (CBDCA) in BALB/c and C57B1/6 mice. On a weekly schedule, i.p. injection of 200 mg/kg WR-2721 at 5 min prior to the administration of this combination enabled us to increase the CBDCA dose from a nontoxic level of 45 mg/kg to a normally toxic dose of 60 mg/kg in non-tumor-bearing BALB/c mice while maintaining the 5FU dose at 100 mg/kg. When WR-2721 was given 30 min before this combination, the CBDCA dose could not be increased to 60 mg/kg without producing drug-related deaths. WR-2721 protected against CBDCA- and 5-FU-induced thrombocytopenia but did not prevent leukopenia or anemia in C57B1/6 mice. The antitumor activity of the combination against colon 26 tumors in BALB/c mice was increased by pretreatment with WR-2721, which facilitated elevation of the CBDCA dose to 60 mg/kg in combination with 100 mg/kg 5FU. These results reveal better therapeutic efficacy for the combination of 5FU and CBDCA following pretreatment with WR-2721.