Although B cells produce cytokines it is not known whether B cells can differentiate into effector subsets that secrete polarized arrays of cytokines. We have identified two populations of "effector" B cells (Be1 and Be2) that produce distinct patterns of cytokines depending on the cytokine environment in which the cells were stimulated during their primary encounter with antigen and T cells. These effector B cell subsets subsequently regulate the differentiation of naïve CD4+T cells to TH1 and TH2 cells through production of polarizing cytokines such as interleukin 4 and interferon γ. In addition, Be1 and Be2 cells could be identified in animals that were infected with pathogens that preferentially induce a Type 1 or Type 2 immune response. Together these results suggest that, in addition to their well defined role in antibody production, B cells may regulate immune responses to infectious pathogens through their production of cytokines.