It is shown that partial phenotypic suppression of two ochre mutations (argE3 andlacZU118) and an amber mutation (inargE) by sublethal concentrations of streptomycin in anrpsL⁺ (streptomycin-sensitive) derivative of theEscherichia coli strain AB1157 greatly enhances their adaptive mutability under selection. Streptomycin also increases adaptive mutability brought about by theppm mutation described earlier. Inactivation ofrecA affects neither phenotypic suppression by streptomycin nor replication-associated mutagenesis but abolishes adaptive mutagenesis. These results indicate a causal relationship between allele leakiness and adaptive mutability.