TGF-β family cytokines play multiple roles in immune responses. TGF-β1-null mice suffer from multi-organ infiltration that leads to their premature death. T cells play a central role in the TGF-β1 phenotype, as deficiency of TGF-β1 only in T cells reproduces the lethal phenotype. Although it is known that TGF-β1 controls B cells isotype switch and homeostasis, the source responsible for this control has not been characterized. Because of the major role that T cells play in regulating B cell responses, we addressed the T cell dependency of the TGF-β1 control of B cells. The analysis of T cell-deficient, TGF-β1 knockout mice and the production of chimeras in which B but not T cells lacked TGF-β1 allowed us to show that B cells are controlled in part by cell autonomous production of TGF-β1.