We have previously isolated seven mutants of Escherichia coli which replicate their DNA with increased fidelity. These mutants were isolated as suppressors of the elevated mutability of a mismatch-repair-defective mutL strain. Each mutant was shown to contain a single amino acid substitution in the dnaE gene product, the alpha (i.e., polymerase) subunit of DNA polymerase III holoenzyme responsible for replicating the E. coli chromosome. The mechanism(s) by which these antimutators exert their effect is of interest. Here, we have examined the effects of the antimutator alleles in a mutD5 mutator strain. This strain carries a mutation in the dnaQ gene, which results in defective exonucleolytic proofreading. Our results show that dnaE mutations also confer a strong antimutator phenotype in this background, the effects being generally much greater than those observed previously in the mutL background. The results suggest that the dnaE antimutator alleles can exert their effect independently of exonucleolytic proofreading activity. The large magnitude of the antimutator effects in the mutD5 background can be ascribed, at least in part, to the (additional) restoration of DNA mismatch repair, which is generally impaired in mutD5 strains because of error saturation. The high mutability of mutD5 strains was exploited to isolate a strong new dnaE antimutator allele on the basis of its ability to suppress the high reversion rate of an A.T-->T.A transversion in this background. A model suggesting how the dnaE antimutator alleles might exert their effects in proofreading-proficient and -deficient backgrounds is presented.