Suspension cultures of mouse fibroblasts L-A 115 and Chinese hamster 237-2a fibroblast were used to study combined action of chromium and nickel ions. The growth curve analysis of L-A cell lines revealed a lethal effect of Cr(VI) concentrations 10(-3)-10(-5)M on cultured cells, concentrations 10(-6)-10(-8)M produced a significant growth inhibition. On the other hand, Cr(III) concentrations 10(-3)-10(-5)M had no effect on cellular growth, which confirms that Cr(VI) is more toxic than Cr(III), even for cultured cells. Ni(II) concentration 10(-3)M was considerably cytotoxic, concentrations 10(-4)-10(-6)M had practically no effect on the cell growth. The combined action of two non-toxic concentrations Ni(II) 10(-5)M X Cr(III) 10(-3)M showed a pronounced of action and produced a 50% growth inhibition in comparison with controls. Surprising and unexpected results were also obtained by the cytogenetic analysis of Chinese hamster 237-2a cells exposed to the combination Ni(II) 10(-6)M X Cr(III) 10(-4)M; the number of detected aberrant cells was significantly lower than the number expected after a simple summation of actions. The reduced mutagenic activity of these Cr(III) and Ni(II) concentrations at their combined action can be ascribed to the stimulation of cellular repair mechanisms.