The aim of this experiment was to test the modulation of genotoxicity produced by vitamin C (V‐C) challenged against mitomycin C (MMQ in vivo, by analyzing the sister chromatid exchanges (SCE) and cell proliferation kinetics. We used the mouse bone marrow cytogenetic method, and tested three dosages of V‐C (3, 5, and 7 g/kg of body weight), along with the appropriate positive (2 mg MMC/kg body weight) and negative V‐C controls. The results showed that V‐C caused a strong inhibition of SCEs induced by MMC in the three dosages administered. The highest dose (7 glkg) caused an SCE inhibition of 70.02%, while the lower ones caused an inhibition of 54.61% and 52.30%, respectively. It was also clear that V‐C per se does not increase the level of SCEs in mouse bone marrow cells. On the other hand, V‐C induced a slight decrease in cell proliferation that was stronger when combined with MMC. Our data suggest that V‐C effectively inhibit the SCE damage in vivo, but caution must be taken because of the observed cytotoxicity.