TE44, an H-2b-restricted, self-reactive T-cell line, did not produce autocrine-acting growth factors, neither after antigenic nor after mitogenic activation; they remained for their proliferation completely dependent on exogenously added IL-2. Administration of IL-4, which poorly promotes growth by itself, resulted in a 5- to 10-fold enhancement of the specific biological activity of IL-2 on antigen-activated TE44-cells. This synergism was exerted nonreciprocally and required the presence of both lymphokines. IL-4 did not affect the number, nor the affinity, nor the rate of internalization of the high-affinity receptors for IL-2. However, increased levels of intracellular IL-2 were observed, suggesting an effect of IL-4 on the turnover of IL-2. This might allow a prolonged activity of IL-2 or IL-2-associated molecules inside the cell. Furthermore, the lack of autocrine growth factor production by antigen-stimulated TE44 is discussed in terms of its relationship to the autoimmune specificity of these T-cells.