Hematopoietic cytokines, including interleukin-3 (IL-3), IL-6, and granulocyte colony-stimulating factor (G-CSF), induce the proliferation, differentiation, and activation of hematopoietic lineage cells. These cytokines activate the Jak/Stat-mediated signal transduction pathway that is important in the biologic activities of these cytokines. In this study, we showed that hematopoietic cytokines, such as IL-3, IL-6, and G-CSF, all induced tyrosine-phosphorylation of Stat family proteins and Stat-associated 150-kD and 72-kD molecules in hematopoietic lineage cell lines. Furthermore, we showed that the 72-kD molecule had tyrosine kinase activity. The tyrosine kinase activity of the 72-kD molecule was enhanced by the stimulation through an IL-6 signal transducer, gp130, that was shared among the receptors for the IL-6-related cytokine subfamily, such as leukemia inhibitory factor, oncostatin M, IL-11, and ciliary neurotrophic factor. Because 72-kD tyrosine kinase was distinct from Syk, Tec, and Btk and coimmunoprecipitated with anti-Stat antiserum, we termed it Stat- associated 72-kD tyrosine kinase (p72sak). p72sak may directly activate Stat family proteins or other signal transducing molecules for IL-3, G- CSF, and the IL-6-related cytokine subfamily.