T ransforming growth factor-p is a pleiotropic cy-tokine having suppressive effects on growth and both inhibitory and stimulatory effects on the differentiation of a variety of cell types. It is an important immunomodulator, with both inductive and suppressive effects on inflammation and the immune response. It also plays an important role in fibrinogenesis and wound repair (1-3). Mice genetically deficient in the gene for TGF-P1, generated by knockout technology, die from a wasting syndrome accompanied by widespread inflammation (4-6). This review will focus on the effect of TGF-P on B cell differentiation, ie, its ability to direct class switching to IgA and IgG2b, and its stimulatory and inhibitory effects on Ig secretion. I will attempt to clarify the basis for the sometimes contradictory effects of TGF-P, and point out areas in which further research is required.