We report the effects of chromium picolinate (CrPic) on micronucleus frequency, morphology of lymphocytes, and lipid peroxidation in calves. Twenty-four Holstein calves were selected for the study. They were kept in a farm and were fed a commercially available calf diet and alfalfa, ad libitum. The animals were divided into three groups of eight subjects each and were treated as follows: The first group was supplemented with a daily dose of 200 mu g Cr as chromium picolinate; a second group received 400 mu g Cr per day and a third group that served as control received no supplemental chromium. After 12-week supplementation, blood samples were collected to determine the micronucleus frequency, the apoptotic cell percentage, and the malondialdehyde (MDA) and blood chromium levels. In both supplemented groups, the cells had irregularly shaped and segmented nuclei. Supplementation also increased the percentage of apoptotic cells (p<0.001) and serum MDA (p<0.01) and slightly increased the chromium levels. The animals supplemented with 400 mu g showed a significant increase of micronucleus frequency (p<0.01). The results of this study suggest that supplementation with 200 and 400 mu g chromium as chromium picolinate may lead to cytotoxicity. The higher level of supplementation may also have genotoxic effects. However, further studies investigating the mechanism of the action of CrPic are required.