Activation of early response genes by interferons (IFNs) and other cytokines requires tyrosine phosphorylation of a family of transcription factors termed signal transducers and activators of transcription (Stats). The Janus family of tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) is required for cytokine-induced tyrosine phosphorylation and dimerization of the Star proteins. In order for IFNs to stimulate maximal expression of Stat1α- regulated genes, phosphorylation of a serine residue in the carboxy terminus by mitogen-activated protein kinase (MAPK) is also required. In HeLa cells, both IFN-β and oncostatin M (OSM) stimulated MAPK and Raf-1 enzyme activity, in addition to Stat1 and Stat3 tyrosine phosphorylation. OSM stimulation of Raf-1 correlated with GTP loading of Ras, whereas IFN-β activation of Raf-1 was Ras independent. IFN-βand OSM-induced Raf-1 activity could be coimmunoprecipitated with either Jak1 or Tyk2. Furthermore, HeLa cells lacking Jak1 displayed no activation of STAT1α, STAT3, and Raf-1 by IFN-β or OSM and also demonstrated no increase in the relative level of GTP-bound p21(ras) in response to OSM. The requirement for Jak1 for IFN-β- and OSM- induced activation of Raf-1 was also seen in Jak1-deficient U4A fibrosarcoma cells. Interestingly, basal MAPK, but not Raf-1, activity was constitutively enhanced in Jak1-deficient HeLa cells. Transient expression of Jak1 in both Jak-deficient HeLa cells and U4A cells reconstituted the ability of IFN-β and OSM to activate Raf-1 and decreased the basal activity of MAPK, while expression of a kinase-inactive form of the protein showed no effect. Moreover, U4A cells selected for stable expression of Jak1, or COS cells transiently expressing Jak1 or Tyk2 but not Jak3, exhibited enhanced Raf-1 activity. Therefore, it appears that Jak1 is required for Raf-1 activation by both IFN-β and OSM. These results provide evidence for a link between the Jaks and the Raf/MAPK signaling pathways.