Peripheral blood leucocytes of healthy donors, both intact and mutagen-processed with cadmium chloride, dioxidine, and bleomycin in vitro, were studied with cytogenetic methods in G2 cell cycle period before, during, and after 14- and 30-day oral administration of vitamin and vitamin-mineral complexes. The study found that intake of complexes containing vitamins in total doses exceeding day requirements did not increase the rate of spontaneous mutations and decreased the sensitivity of human cells to the cytogenetic activity of chemical mutagens. At the same time, vitamin complexes of certain quantitative and qualitative composition were shown to be capable of giving rise to comutagenic modification of sensitive cells to the action of separate mutagens in certain concentrations in vitro. The above effects of anti- and comutagenic modification depend on the duration of intake, the quantitative and qualitative composition of the vitamin complex, the nature of mutagens, and the level of induced cytogenetic lesions.