Transforming growth factor β (TGFβ) has potent immunoregulatory effects acting on both T and B cells. It strongly inhibits secretion of IgG and IgM in human and murine B cell cultures, but has been shown to have an enhancing effect on IgA production in the mouse. We have studied the effect of TGFβ on the production of IgA in human lymphocyte cultures. The addition of TGFβ to pokeweed-stimulated peripheral blood lymphocytes resulted in a suppression of IgA production of both subclasses, similar in magnitude to the suppression of IgG and IgM production. Membrane IgA expression was not increased by culturing tonsillar lymphocytes with TGFβ. In conclusion, we find no evidence for a selective enhancing effect of TGFβ on IgA synthesis in humans, in contrast to the findings reported in mice. © 1992.