Batf belongs to the activator protein 1 superfamily of basic leucine zipper transcription factors that includes Fos, Jun, and Atf proteins. Batf is expressed in mouse T and B lymphocytes, although the importance of Batf to the function of these lineages has not been fully investigated. We generated mice (Batf^ΔZ/ΔZ) in which Batf protein is not produced. Batf^ΔZ/ΔZ mice contain normal numbers of B cells but show reduced numbers of peripheral CD4⁺ T cells. Analysis of CD4⁺ T helper (Th) cell subsets in Batf^ΔZ/ΔZ mice demonstrated that Batf is required for the development of functional Th type 17 (Th17), Th2, and follicular Th (Tfh) cells. In response to antigen immunization, germinal centers were absent in Batf^ΔZ/ΔZ mice and the maturation of Ig-secreting B cells was impaired. Although adoptive transfer experiments confirmed that this B cell phenotype can be driven by defects in the Batf^ΔZ/ΔZ CD4⁺ T cell compartment, stimulation of Batf^ΔZ/ΔZ B cells in vitro, or by a T cell–independent antigen in vivo, resulted in proliferation but not class-switch recombination. We conclude that loss of Batf disrupts multiple components of the lymphocyte communication network that are required for a robust immune response.