SUMMARY A comparison has been made between the serological properties of 35 strains of Lactobacillus casei and the carbohydrate components of corresponding cell-wall preparations. All strains of L. casei var. rhamnosus belonged to serological group C, whereas strains of L. casei var. casei belonged to group B or C. Rhamnose was the major cell-wall component of group B strains, galactose glucose being minor variable components. Of the group C strains only those classified as L. casei var. rhamnosus contained significant amounts of rhamnose in the cell wall. All strains in group C, however, contained galactose and glucose as major cell-wall components.