Rodent models have been extensively utilized to identify putative human immunotoxicants; however, even when immunotoxicity is established, uncertainty remains whether the effects are predictive of human risk. Therefore, the objective of this study was to establish a polyclonal immunoglobulin M (IgM) antibody-forming cell (AFC) response model to directly characterize immunotoxicity in primary mouse or human B cells. CD40 ligand (CD40L) was selected to activate B cells because it effectively drives both primary human and mouse B cells in vitro to AFC in a physiologically relevant manner to mimic T-cell-dependent antibody responses in vivo. In this model, the IgM AFC response is induced by cell surface-expressed CD40L and promoted by recombinant cytokines. Reported here are the conditions required to induce IgM AFC responses using mouse splenic B cells or human peripheral blood B cells, allowing for species comparisons. Moreover, less than one order of magnitude difference was observed in the CD40L-induced B-cell AFC responses based on data from multiple donors. In addition to antibody production, proliferation and phenotypic changes characteristic of B-cell activation as well as the plasma cell phenotype were also significantly induced. Finally, two well-characterized immunotoxicants, arsenic and benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide, using the CD40L-induced IgM AFC response were compared in both mouse and human B cells. Collectively, an IgM AFC response model is described that can be applied to assess the sensitivity of antibody responses to modulation by xenobiotics using mouse as well as human primary B cells.