CD70, the cellular ligand of the tumor necrosis factor receptor family member CD27, can be found on a limited number of germinal center (GC) B cells in some tonsils, on scattered lymphocytes residing in secondary lymphoid organs, and on a fraction of the circulating B cell population. Due to the restricted expression of CD70 in vivo, we analyzed signals that determine CD70 expression levels and characterized the phenotype and function of CD70+ B cells. Expression of CD70 on B cells activated in vitro was found to be dependent on the continuous presence of a B cell antigen receptor cross-linking agent, and induced or potentiated by CD40 ligation but was down-modulated by the Th2 cytokines interleukin (IL)-4 and IL-13. Both in peripheral blood and tonsil cell suspensions, CD70+ B cell subpopulations were found to be enriched for CD27- and IgG-expressing cells, but contained less IgD+ B cells. Additional analysis of markers which define specific differentiation stages (Bm1-5) of mature B cells within human tonsils did not place CD70-expressing B cells in one of these subsets. Functional experiments revealed that whereas both CD70- and CD70+ B cells can secrete immunoglobulin after activation with a combination of Staphylococcus aureus Cowan strain I and IL-2, only CD70+ B cells can produce large quantities of antibodies when stimulated in a T cell-dependent fashion. Our combined data imply that CD70 is a marker for mature B cells which have recently been primed by antigen in vivo.