Mutagenesis in the lacI gene of Escherichia coli has been examined in cells grown in the presence of beryllium, manganese or chromium compounds, metals with suspected mutagenic or carcinogenic potential. 2--3-fold increases in mutation frequency were produced by BeCl2, MnCl2 and K2Cr2O7. Among the cells grown in the presence of Be2+, the frequency of amber and ochre mutants was 3-fold higher than the spontaneous background, suggesting that at least part of the increased mutagenicity was due to base-substitution mutations. The specificity of base-substitution mutations induced by Be2+ and Mn2+ in the lacI gene was analyzed. Among the amber mutations induced in cells grown in the presence of Be2+, an increase in G:C----A:T transitions was detected. In contrast, following growth in Mn2+, no increase in amber and ochre mutation frequencies was observed, and the mutational spectrum resembled that obtained spontaneously indicating that mutations induced by Mn2+ in the lacI gene involve changes that do not yield nonsense mutations. These results suggest that metals may exert a number of different mutagenic effects and that these effects vary for each metal.