Abstract When eight phage T4 rII base substitution mutations were each combined with an antimutator allele of gene 43 (DNA polymerase), reversion frequencies of the rII mutations were reduced from 3- to 90-fold. When these eight rII-gene 43 allele combinations were then combined with an allele of gene 32 (helix-destabilizing protein), the low reversion index was maintained in five cases. In the remaining three cases, strong mutator effects by the gene 32 allele were observed. One of these three rII alleles was then used as a tester in combination with an allele from each of genes 30, 37, 39, 41, 44, 45, 47, and 56. The corresponding triple mutants, consisting of the rII allele, the gene 43 antimutator allele, and an allele from each of the above genes was also constructed. The alleles of genes 41, 44, and 45 displayed strong mutator effects in the presence of the gene 43 antimutator polymerase and much weaker mutator effects in the presence of wild-type polymerase. The tested alleles of genes 30, 37, 39, 47, and 56 did not have such mutator effects. The genes whose alleles showed strong mutator effects (32, 41, 44, and 45) have been identified previously as being essential for DNA synthesis in vivo, and for a rapid rate of replication in vitro. It has recently been hypothesized that these gene products and the products of genes 61 and 62 may interact with the gene 43 DNA polymerase to form a multienzyme replicative complex which governs the accuracy of replication. The data presented here, together with previously reported results on mutator effects, support the hypothesis that all seven gene products of the replicative complex have a strong influence on the accuracy of replication.