We have shown previously that tumor necrosis factor-alpha (TNF-alpha) inhibits the growth and induces the differentiation of a murine myelomonocytic leukemia (WEHI-3B JCS cells) into macrophage-like cells. In this study, using reverse transcription polymerase chain reaction, we found that both endogenous interleukin-1 alpha and beta (IL-1 alpha, IL-1 beta) mRNA were up-regulated upon TNF-alpha induction. Exogenous IL-1 alpha and IL-1 beta also inhibited the growth as well as induced the differentiation of JCS cells, with IL-1 beta exerting a greater growth-inhibitory effect. Neutralizing anti-IL-1 alpha, anti-IL-1 beta and anti-TNF-alpha antibodies were further used to elucidate the role of IL-1 alpha, IL-1 beta and TNF-alpha in JCS cell differentiation. The results show that the IL-1 alpha-induced monocytic differentiation of JCS cells was effectively blocked by anti-IL-1 beta as well as anti-IL-1 beta antibodies and to a lesser extent by anti-TNF-alpha antibody. In contrast, the differentiation-inducing effect of IL-1 beta on JCS cells was only blocked by anti-IL-1 beta antibody but not by anti-IL-1 alpha or anti-TNF-alpha antibody. Finally, the TNF-alpha-induced monocytic differentiation of JCS cells was significantly blocked by anti-TNF-alpha and to a lesser extent by anti-IL-1 alpha and anti-IL-1 beta antibodies. Collectively, our results suggest that IL-1 beta alone may directly trigger JCS cell differentiation whereas the differentiation-inducing effect of IL-1 alpha may be via the endogenous production of IL-1 beta and/or TNF-alpha. In addition, IL-1 alpha and IL-1 beta may be involved, at least in part, in TNF-alpha-induced monocytic differentiation of the JCS leukemia cells.