Subpopulations of mouse B cells express different amounts of two antigens (BLA-1 and BLA-2) recognized by rat monoclonal antibodies (53-10.1 and 30-E2). Two-color immunofluorescence analysis on the fluorescence-activated cell sorter (FACS) shows that the 53-10.1 monoclonal antibody reacts with a similar proportion of splenic B cells from normal and CBA/N (xid) mice, whereas 30-E2 reacts with most CBA/N B cells but with only a fraction of normal B cells. Data from three- and four-color immunofluorescence analyses with xid, athymic (nude), and normal mice suggest that the order in which these antigens are lost during B cell differentiation distinguishes two B cell lineages: immature B cells express both antigens, intermediate-stage B cells of one or the other lineage express only BLA-1 or only BLA-2, respectively, and mature resting B cells express neither. CBA/N mice lack one of the putative intermediate populations (BLA-1+,2-); thus, this population apparently gives rise to the predominant mature B cell population, which is present in normal adult spleen and lymph node but is missing in CBA/N. The other putative intermediate population (BLA-1-,2+) is decreased by two- to threefold in spleens from nude mice compared with strain-matched controls. Both BLA-1 and BLA-2 antigens rapidly reappear after specific (antigen) or nonspecific (lipopolysaccharide) B cell activation. IgM plaque-forming cells (PFC) derived from such activated cells continue to express both antigens while IgG PFC express only BLA-1.