Escherichia coli K12 strain, FC40, has been used extensively in the analysis of adaptive mutability. This strain carries a revertible mutant lac allele on an F plasmid and accumulates Lac(+) (lactose utilizing) revertants, but not unselected mutants, when placed on selective medium. These adaptive mutations are a subset of spontaneous types and their formation depends on the RecABC functions. Data presented here suggest that this phenomenon depends on transfer functions of the F factor. Fertility inhibition eliminates RecA-dependent adaptive reversion. Thus, "adaptive" revertants may form during replication from the transfer origin, whereas loci in the nonreplicating chromosome show little mutation.