AbstractWe have shown that tumor necrosis factorα(TNFα) and interleukin 2 (IL-2) are complementary for stimulation of growth of the hematopoietic stem cell line, LyD9. Neither TNFαnor IL-2 alone could stimulate the proliferation of LyD9 cells even after pre-incubation with these growth factors. The number of high-affinity IL-2 receptors on LyD9 cells did not increase after incubation with IL-2 and TNFα. These results suggest that the proliferative response of LyD9 by TNFαand IL-2 is not mediated by receptor inducing activities. We used the induction of the proto-oncogenesc-mycandc-pimto characterize the proliferative stimulation by IL-2 and TNFα. Northern blot analysis revealed that the simultaneous addition of IL-2 and TNFαwas more efficient than IL-2 alone forc-mycmRNA induction. However, the addition of TNFa and IL-2 could not increasec-pimmRNA more than the level induced by IL-2 alone. The results indicate that the two growth factors complement each other by transducing different types of growth signal.