Antimutators are mutant strains that have reduced mutation rates compa
red to the corresponding wild-type strain. Their existence, along with
mutator mutants that have higher mutation rates compared to the wild-
type strain, are powerful evidence that mutation rates are genetically
controlled. Compared to mutator mutants, antimutators have a very dis
tinguishing property. Because they prevent normally occurring mutation
s, they, uniquely, are capable of providing insight into the mechanism
s of spontaneous mutations. In this review, antimutator mutants are di
scussed in bacteriophage T4 and the bacterium Escherichia coli, with r
egard to their properties, possible mechanisms, and implications for t
he sources of spontaneous mutations in these two organisms.