For the removal of galactose inhibition, the predicted galactose binding residues, which were determined by sequence alignment, were replaced separately with Ala. The activities of the Ala-substituted mutant enzymes were assessed with the addition of galactose. As a consequence, amino acid at position 349 was correlated with the reduction in galactose inhibition. The F349S mutant exhibited the highest activity in the presence of galactose relative to the activity measured in the absence of galactose among the tested mutant enzymes at position 349. The K (i) of the F349S mutant (160 mM), which was 13-fold that of the wild-type enzyme, was the highest among the reported values of beta-galactosidase. The wild-type enzyme hydrolyzed 62% of 100 g lactose/l with the addition of 30 g galactose/l, whereas the F349S mutant hydrolyzed more than 99%.