The natural adherence of eight strains of bacteria to mouse spleen cells is described. The bacteria used were: Arizona hinshawii, Brucella abortus, Brucella melitensis, Corynebacterium xerosis, three strains of Escherichia coli (E. coli-2, E. coli-3 and E. coli-4), and Pasteurella pestis. A strain of E. coli that did not bind naturally to spleen cells was chemically coated with anti-mouse Ig antibodies (anti-Ig Ab-E. coli) and used to identify cells with surface Ig, i.e., Ig+ cells (B cells). Cells that lacked detectable surface Ig (Ig- cells) were considered to be primarily T cells. By mixing anti-Ig Ab-E. coli and another bacteria with the lymphocytes, we found that some of the bacteria bound to either Ig+ or Ig- cell subsets and some bound to both Ig+ and Ig- cell subsets. One strain of bacteria (B. melitensis) bound to essentially all of the Ig+ cells and none of the Ig- cells. By labeling the lymphocytes with various combinations of two bacteria, we identified three Ig+ (B cell) and three Ig- (presumably T cell) lymphocyte subpopulations. B1 cells bound B. melitensis, B. abortus, and C. xerosis; B2 cells bound B. melitensis, A. hinshawii, C. xerosis, E. coli-2, and E. coli-4; B3 cells bound B. melitensis, P. pestis, and E. coli-2, T1 cells bound C. xerosis, E. coli-2, and E. coli-3; T2 cells bound E. coli-3 and E. coli-4. T3 cells did not bind any of the bacteria. Whereas B. melitensis, C. xerosis, E. coli-2, E. coli-3, and E. coli-4 adhered to more than one subset of lymphocytes, B. abortus, A. hinshawii, and P. pestis adhered to only a single subset—B1, B2, and B3 cells, respectively. The use of the natural adherence of bacteria to lymphocytes provides a simple method for identifying lymphocyte subpopulations and may facilitate the elucidation of the roles that lymphocyte subsets play in immune phenomena.