Streptococcus lactis UN possessed galactokinase, uridine diphosphate glucose epimerase (galacto-waldenase), hexokinase, aldolase, lactic dehydrogenase, alcohol dehydrogenase, and aldehyde dehydrogenase enzyme systems. The presence of galactokinase, galactowaldenase, and hexokinase indicated that monosaccharides, galactose, and glucose were phosphorylated and converted into glucose-6-phosphate, which then enters the glycolytic cycle. Galactokinase and UDPG epimerase enzymes were found to be adaptive in nature. However, hexokinase, aldolase, and dehydrogenases for lactate, alcohol, and aldehyde were found to be constitutive in nature, as the organism, regardless of the substrate used, always possessed these five enzymes. Both alcohol and aldehyde dehydrogenases were NAD-specific rather than NADP-specific.Penicillin, streptomycin, aureomycin, and terramycin inhibited the activities of these enzyme systems to varying degrees. Also, penicillin and streptomycin inhibited the production of galactokinase and UDPG epimerase by the organism, but had no effect upon the production of hexokinase.