The activation of Janus protein tyrosine kinases (Jak) and STAT (signal transducer and activator of transcription) proteins has recently been linked to the signal transduction mechanism of several cytokines. IL‐7 was observed to induce a rapid and dose‐dependent tyrosine phosphorylation of Jak 1 and Jak 3 and concomitantly, the tyrosine phosphorylation and DNA binding activity of multiple STAT proteins. The STAT proteins utilized by IL‐7 were identical to those induced by IL‐2 and could be identified as various STAT 5 isoforms. Moreover, the induction of both Jak 1 and 3, and STAT 5 activity strongly correlated with the growth‐promoting effects of IL‐7, suggesting that this signal transduction mechanism may play a key role in IL‐7‐induced proliferation.