In the last decade, there is an increasing interest in selenium compounds because of their environmental, biological, and toxicological importance, and in particular, because of their anticancer activities. We have examined the effect of sodium selenite on colony formation of different human cultured malignant cells. In addition, the antimutagenic activities of this chemical was examined against the indirect-acting mutagen benzo[a]pyrene (BP) by using the Ames/Salmonella test assay TA98 strain. Results of these experiments demonstrated that sodium selenite caused a dose-dependent inhibitory effect on colony formation of tested malignant cells. It was also shown that sodium selenite exhibited dose-dependent antimutagenic effect on BP. These results suggest the potential use of selenium compounds as an anticancer and antimutagenic agent.