In vivo clastogenic effects of mitomycin-C (MMC) in bone marrow cells of four groups of young male Swiss albino mice exposed to 0, 7.5, 15, and 30 mg NaF/L in their drinking water for 30 days were investigated. The percentages of aberrant metaphases and chromosome aberrations in all F-treated mice were significantly increased. In another experiment, NaF pretreatment was followed by intraperitoneal administration of 4 mg MMC/kg bw. Results indicated that the two chemicals did not have a synergistic effect; instead, a notable suppression of the clastogenic effect of MMC occurred. As expected, mitotic indices (MI) in the bone marrow cells of the MMC-treated mice were significantly lower than in the controls. However, in the mice pretreated with 30 mg NaF/L, the percent of MI was greater than in the MMC-treated group without NaF exposure. Moreover, the percentages of aberrant metaphases and chromatid breaks were significantly higher in all the F groups than in the controls. NaF exposure of the mice for 30 days evidently also helped prevent MMC-induced damage, although the effect was not statistically significant. In the mice pre-exposed to 30 mg NaF/L in their drinking water without treatment with MMC, a 10% decrease in chromatid breaks was observed. Thus these in vivo findings corroborate earlier reports of clastogenic effects of NaF treatment in vitro.