Ni2+ ions exhibit a mutagenic effect on the bacterial strain Corynebacterium sp. 887 (hom). The mutagenic activity of the divalent nickel was demonstrated by both the simplified fluctuation test and the so-called clone method. However, when using the clone method and low nickel concentrations the frequency of induced mutants decreases considerably as compared with the control and Ni2+ ions have an antimutagenic effect under these conditions.