gp39, the ligand for CD40 expressed on activated CD4+ T helper cells, is required for the generation of antibody responses to T-dependent (TD) antigens. Treatment of mice with anti-gp39 in vivo inhibits both primary and secondary antibody formation to TD, but not T-independent antigens. However, the role of this receptor-ligand pair in the development of germinal centers and the generation of B cell memory is as yet undefined. Using an antibody to gp39, this study examines the in vivo requirement for gp39-CD40 interactions in the induction of germinal center formation, as well as in the generation of B cell memory. Animals were immunized, treated in vivo with anti-gp39, and evaluated using immunohistochemical staining for the presence of splenic germinal centers 9-11 d after immunization. The results demonstrate that the formation of germinal centers was completely inhibited as a result of treatment with anti-gp39. Moreover, adoptive transfer experiments demonstrate that the generation of antigen-specific memory B cells is also inhibited as a consequence of blocking gp39-CD40 interactions. Taken together, the data demonstrate that gp39-CD40 interactions are critical not only for the generation of antibody responses, but also in the development of B cell memory.