The metabolism of sucrose and lactose by Streptococcus mutans and Lactobacillus casei were studied. Intracellular invertase was shown to be a sucrose-6-phosphate hydrolase in strains of S. mutans representative of the seven serotypes. The purified S6P hydrolases appear similar in these strains and differed in properties from the extracellular invertase. An ATP dependent mannofructokinase was further characterized; high yields of a stable preparation are now available to study the role of the enzyme in intracellular phosphorylation of fructose. Beta-galactoside metabolism (e.g. lactose, lactulose, lactobionate, methyl-beta-galactoside, arabinosyl-beta-galactoside) was mediated by plasmid-determined lactose-PTS an P-beta-galactosidase activity in L. casei. Restriction enzyme analysis of several of the lactose plasmids indicated that they share functional, but not structural homology. Using mutanolysin, methods were developed for the generation of viable spheroplasts of L. casei. These spheroplasts are being used to study transformation and cell-cell fusion in order to facilitate genetic transfer among Lactobacilli. Other data indicate that L. casei possess a number of distinct PEP-dependent sugar-specific PTS activities which are currently being characterized.