With two new monoclonal antibodies and flow cytometry, we defined three subpopulations among B cells expressing binding sites for peanut agglutinin (i.e., B cells of the germinal center). One monoclonal antibody (5B5) binds globotriaosyl ceramide. The B lymphocytes binding 5B5 have binding sites for peanut agglutinin on the surface and express only small amounts of sIgD and sIgM. When tested against a panel of B cell lines, only Burkitt's lymphoma cells were 5B5+. Moreover, the 5B5+ cells have larger average sizes and a large fraction of proliferating cells. The other monoclonal antibody (HK23) binds a 90,000 protein. Lymphocytes binding HK23 are 5B5- and include T cells and a subpopulation of B cells. In contrast to 5B5+ cells, the HK23+ and peanut agglutinin positive B cells express a large amount of sIgM. These two subpopulations of germinal centers are distinct from the germinal center B cell subpopulation expressing the CD23 (Blast-2)antigen. The CD23+ B cells are 5B5- and express an intermediate level of HK23 antigen. In addition, CD23+ B cells are highly variable in number, whereas the proportions of HK23+ and 5B5+ cells are relatively stable.