The CD80/86-CD28 and CD40-CD40 ligand costimulatory pathways are essential for Th cell-dependent B cell responses that generate high-affinity, class-switched Ab in vivo. Disruption of either costimulatory pathway results in defective in vivo humoral immune responses, but it remains unclear to what extent this is due to deficient activation of Th cells and/or of B cells. To address this issue, we generated mixed chimeras in which CD80/86- or CD40-deficient bone marrow-derived cells coexist with wild-type (WT) cells, thereby providing the functional T cell help and accessory cell functions required for fully competent B cell responses. We were then able to assess the requirement for CD80/86 or CD40 expression on B cells producing class-switched Ig in response to a T-dependent Ag. In CD80/86 WT plus CD80/86 double-knockout mixed chimeras, both WT- and CD80/86-deficient B cells produced IgG1 and IgE responses, indicating that direct signaling by CD80/86 is not essential for efficient B cell activation. In marked contrast, only WT IgG1 and IgE responses were detected in the chimeras containing CD40-deficient cells, demonstrating that CD40 expression on B cells is essential for class switching by those B cells. Thus, while disrupting either the CD80/86-CD28 or the CD40-CD40 ligand costimulatory pathway abrogates T-dependent B cell immune responses, the two pathways are nonredundant and mediated by distinct mechanisms.