B cells provide humoral protection against pathogens and promote cellular immunity through diverse nonclassical effector functions. To assess B cell function in promoting T cell homeostasis, mature B cells were either acutely or chronically depleted in mice using CD20 mAb. Acute B cell depletion in either 2- or 4-mo-old mice significantly reduced spleen and lymph node CD4(+) and CD8(+) T cell numbers, including naive, activated, and Foxp3(+)CD25(+)CD4(+) regulatory T cell subsets. The numbers of IFN-γ- and TNF-α-producing T cells were also significantly reduced. Chronic B cell depletion for 6 mo in aged naive mice resulted in a 40-70% reduction in activated CD4(+) and CD8(+) T cell numbers and 20-50% reductions in IFN-γ-producing T cells. Therefore, B cells were necessary for maintaining naive CD4(+) and CD8(+) T cell homeostasis for subsequent optimal T cell expansion in young and old mice. To determine the significance of this finding, a week of B cell depletion in 4-mo-old mice was followed by acute viral infection with lymphocytic choriomeningitis virus Armstrong. Despite their expansion, activated and cytokine-producing CD4(+) and CD8(+) T cell numbers were still significantly reduced 1 wk later. Moreover, viral peptide-specific CD4(+) and CD8(+) T cell numbers and effector cell development were significantly reduced in mice lacking B cells, whereas lymphocytic choriomeningitis virus titers were dramatically increased. Thus, T cell function is maintained in B cell-depleted mice, but B cells are required for optimal CD4(+) and CD8(+) T cell homeostasis, activation, and effector development in vivo, particularly during responses to acute viral infection.