The protein tyrosine kinases JAK1, JAK2 and Tyk2 and STATs (signal transducers and activators of transcription) 1 and 3 are activated in response to interleukin‐6 (IL‐6) in human fibrosarcoma cells. In mutant cells lacking JAK1, JAK2 or Tyk2, the absence of one kinase does not prevent activation of the others; activation does not, therefore, involve a sequential three‐kinase cascade. In the absence of JAK1, the phosphorylation of the gp130 subunit of the IL‐6 receptor and the activation of STATs 1 and 3 are greatly reduced. JAK1 is also necessary for the induction of IRF1 mRNA, thus establishing a requirement for the JAK/STAT pathway in the IL‐6 response. JAK2 and Tyk2 although activated cannot, in the absence of JAK1, efficiently mediate activation of STATs 1 and 3. A kinase‐negative mutant of JAK2 can, however, inhibit such activation, and ancillary roles for JAK2 and Tyk2 are not excluded. A major role for JAK1 and the nonequivalence of JAK1 and JAK2 in the IL‐6 response pathway are, nevertheless, clearly established for these cells.