Recent studies have demonstrated the importance of regulatory B cells in autoimmune, allergic, and inflammatory diseases. These B cells have an ability to suppress excessive immune reactions by multiple mechanisms. Most studies have focused on IL-10-producing B cells, but we have previously reported that transforming growth factor (TGF)-β1 secretion by B cells also plays an important role in intestinal homeostasis and mucosal inflammation. B cell-secreted TGF-β may be involved in the regulation of T cell immunity (differentiation, proliferation, and apoptosis) that is relevant to the pathogenesis of autoimmune or inflammatory disease. Here, we provide detailed instruction for detecting and measuring TGF-β produced by B cells.