Using the micronucleus test to evaluate the mutagenic effect of 5,5-diphenylhydantoin (DPH) on bone marrow polychromatic erythrocytes, male Balb-C mice were treated with the drug in single and multiple injection tests. A significant increase in the frequency of micronucleated polychromatic erythrocytes (MPE), P less than 0.05, was found when the mice received a single injection of DPH at doses of 0.5 and 1.0 mg/kg, and this frequency did not increase at higher doses. When mice were treated 3 times, at 24-h intervals, with 1.0 mg/kg of DPH, a significant increase in MPE was also observed (P less than 0.05) but this was lower than when they received a single injection of the same dose. A cytotoxic effect of NaOH, 0.1 N, which was used as solvent, was also observed either when alone or when DPH (1.0 mg/kg) was injected 3 times. This effect was comparable to the one produced by mitomycin C (MMC) at a dose of 0.5 mg/kg.