Mutations in the bacteriophage T4 DNA polymerase gene sometimes reduce spontaneous mutation rates, in which case they have been called antimutator mutations. It has gradually become clear that while some kinds of mutations are reduced in frequency in such strains, other kinds of mutations are increased. Both structural and evolutionary considerations suggested that a decrease in the total mutation rate is unlikely to be achievable by a single missense mutation except perhaps at the cost of a reduced rate of DNA synthesis. The average mutation rate over a target of roughly 5 kilobase pairs was determined in a set of 12 antimutator mutations. In each case, the rate was indistinguishable from or greater than the wild-type mutation rate.