The effects of the carcinogenic metal nickel on DNA polymerase alpha (pol alpha) activity and fidelity have been analyzed. In the absence of Mg2+, the presence of Ni2+ ions at concentrations below 0.25 mM gave rise to a dose-dependent activation of pol alpha as monitored by [3H]dTMP incorporation into an activated DNA template. The apparent Km for Ni(2+)-dependent pol alpha incorporation of dTTP was estimated to be 25 microM, which was about 10 times higher than the Km for Mg2+ (2.3 microM). Above 0.25 mM, Ni2+ caused a dose-dependent inhibition of pol alpha activity and the Ki was calculated to be 1.5 mM. Scatchard analyses showed that Ni2+ binds to affinity-purified pol alpha and associated proteins at two tight binding sites with a Kd of approximately 50 microM and at eight weak binding sites with a Kd of approximately 4 mM. In the presence of 2 mM Mg2+, the addition of Ni2+ to the reactions caused an inhibition of polymerase activity. The inhibition patterns tended to switch from competitive to mixed-type to noncompetitive as a function of Ni2+ concentration. Lastly, Ni2+ increased the incorporation of the modified nucleotide dideoxy-CMP in reactions using varying ratios of dideoxy-CTP/dCTP.