Strain BL1003, a lacO mutant, synthesizes beta-galactosidase constitutively at a low rate. The enzyme is further inducible by d-galactose to the same differential rate as is seen in the presence of an optimal concentration of thiomethylgalactoside. lacY Mutants derived from strain BL1003 are not inducible by galactose, although they synthesize beta-galactosidase at the low constitutive rate characteristic of the parent. Galactose is a weak inducer of beta-galactosidase synthesis in wild-type Escherichia coli K-12, but it is more effective when the wild type has been preinduced with isopropyl-beta-d-thiogalactoside. Nevertheless, the rise in the differential rate of synthesis in response to galactose in a preinduced wild-type culture is much lower than in strain BL1003. Thus, two factors are involved in the induction of strain BL1003 by galactose: the mutant operator and the constitutive permease. The operator has an altered sensitivity to the i product-galactose complex. The low constitutive level of permease enabled the cells, at the high concentrations of galactose used (5 x 10(-2)m), to maintain a sufficient internal concentration for further induction.