Several mutator mutants were isolated from a cultured mouse cell line (FM3A). The mutants exhibited a high rate of spontaneous mutation at three genetic loci of drug resistance. To investigate a possible link between mutator phenotype and fidelity of DNA replication, DNA polymerase alpha was partially purified from the wild-type and the mutator mutant (Fmut 1) showing the highest mutation rate. Using a combination of synthetic template and primer, the ratio of incorporation of incorrect to correct nucleotides was determined. The results indicated that the DNA polymerase alpha from the mutator mutant showed a slightly higher rate of misincorporation, 1.4 and 1.6 times, than that of the wild-type.