In this study the three different mammalian isoforms of transforming growth factor-β (TGF-β) were compared with regard to their effect on the response of rat T lymphocytes to the superantigen, staphylococcal enterotoxin A (SEA). All different isozymes were found to increase the proliferative response of rat T lymphocytes, which was accompanied by a significantly lower percentage of apoptotic cells than proliferation in the absence of TGF-β. The same effect of TGF-β was observed on the generation of apoptotic cells in an allo response (mixed lymphocyte reaction). TGF-β2 and TGF-β3 were three to 10-fold more potent than TGF-β1 as co-stimulators of T lymphocytes, and equal in decreasing the percentage of apoptotic T cells. TGF-β1 reduced the frequency and the number of cells undergoing apoptosis in T cells and, to an even higher degree, among B lymphocytes. TGF-β did not seem to affect the production of the apoptosis inducer, tumour necrosis factor-α (TNF-α), neither at the mRNA level nor at the protein level. Neutralizing antibodies against the cytokine, TNF-α, decreased the percentage of apoptotic cells among T cells responding to SEA, both in the absence and in the presence of added TGF-β1. Thus, when TGF-β acts as a co-stimulator for T-cell activation it inhibits the induction of apoptosis and sustains the number of viable cells.