Ochratoxin A (OA), when administered orally daily for 45 days to albino Swiss mice, Mus musculus, at a level equivalent to the human dietary concentration of 1 microgram/kg body weight/day, increased the production of abnormalities in both mitotic and meiotic chromosomes as well as in the gross morphology of the sperm head. The sperm count per unit volume of caput epididymal suspension also decreased. Vitamin C at a concentration equivalent to the human therapeutic dose (10 mg/kg body weight/day), when administered orally concurrently with OA, significantly minimized the incidence of these abnormalities. The protective effect of vitamin C was most marked in mitotic chromosomes followed by that in meiotic chromosomes and sperm head morphology; the improvement in sperm count was least marked. The possible mechanism of this effect is discussed.