WR-2721 (ethiofos) was tested on Balb/c mice for its chemoprotective capacity against 5-fluorouracil (5FU) monotherapy. In this combination WR-2721 was not active, but WR-2721 pretreatment allowed an elevation of the cisplatin (CDDP) dose in 5FU/CDDP combination therapy in these mice. Thrombocytopenia caused by the 5FU/CDDP (100 and 7 mg/kg, respectively) therapy was prevented by WR-2721 (200 mg/kg) and a partial protection against leukopenia was observed in C57Bl/6 mice. Various WR-2721/CDDP/5FU combinations were tested on two murine colon tumour models. The best antiproliferative effect against Colon 26 (in Balb/c mice) and the lowest toxicity were found with 5FU (100 mg/kg) and CDDP (5.5 mg/kg) delivered together 30 min after WR-2721 (200 mg/kg). The increased efficacy of WR-2721/CDDP/5FU both in Colon 26 and Colon 38 (in C57Bl/6 mice) compared to single 5FU or 5FU/CDDP treatment at the same dose could not be explained by enhanced inhibition of thymidylate synthase (TS), the 5FU target enzyme. The protection by WR-2721 against toxicity of CDDP/5FU might enable the use of high doses of CDDP in this combination.