Members of the genus Lactobacillus were classified serologically into several groups by the extensive studies of Sharpe [12][13][14]. She showed that serological specificities were determined by the carbohydrate moiety of the bacterial cells and that L. casei strains were divided into group B and group C.Immunochemical investigations on the specific antigens of groups B and C were carried out by Knox and his coworkers [3][4][5][6][7][8].The group-specific antigens were identified as polysaccharide components of the bacterial cell walls, and the probable configurations of the determinant sites were also presented.However, in addition to these groupspecific antigens, antigens common to both groups were also anticipated, because strong cross reactions were recognized in precipitin reactions using Sharpe's antigens.In our experiments more precise antigenic analyses of L. casei were carried out and some new findings were obtained. MATERIALS AND METHODSOrganisms. Bacterial strains used in the experiments are listed in Table 1. Five strains were kindly supplied by Dr. M. E. Sharpe, National Institute for Research in Dairying, Reading, England, and eight strains were from Dr. T. Mitsuoka, Institute of Physical and Chemical Research, Tokyo, Japan. Remaining four strains were isolated and maintained in our laboratory. Serological classifications into group B or C were carried out by the Sharpe method [12].Preparation of cell walls. Cells grown in the MRS medium [2] for 18 to 24 hr at 37 C were centrifuged at 8000 rpm for 15 min and washed twice with saline. The washed cells were disrupted by grinding with synthetic Zeolite type 4A as described by Wistreich et al [15]. The crude cell walls thus obtained were treated with trypsin and ribonuclease according to the method of Cummins and Harris [1], and washed thoroughly with distilled water.