Interaction of IL-3 with its receptor is known to activate STAT-3 via phosphorylation of Tyrosine 701, which facilitates its dimerization and translocation to the nucleus, leading to the transcription of its target genes. In this communication, we have investigated the nature of tyrosine kinases that mediate STAT-3 phosphorylation during IL-3-mediated activation of myeloid cell proliferation. Our results show that interaction of IL-3 with its receptor leads to the activation of c-Src kinase activity, which in turn facilitates the binding of c-Src to STAT-3. This association leads to the phosphorylation of STAT-3, allowing this transcription factor to translocate to the nucleus. Expression of a dominant negative mutant of src (AMSrc) in these cells results in a block to IL-3 mediated phosphorylation of STAT-3, and its ability to bind to DNA. On the other hand, expression of a dominant negative mutant of JAK2 (JAK2KE) had no effect on IL-3-mediated activation of STAT-3. Our results also show that AMSrc does not affect the phosphorylation of JAK2, suggesting that JAK and STAT phosphorylation events are mediated by two independent pathways. Inhibition of c-Src activation by AMSrc, which leads to a block to STAT-3 activation, results in a dramatic inhibition of cell proliferation mediated by IL-3. However, expression of AMSrc does not activate apoptotic pathways. In contrast, expression of JAK2KE results in accelerated apoptosis of 32Dcl3 cells grown in the absence of IL-3 with concomitant down-regulation of Erk-2 kinase activity. These results suggest that Src family kinases mediate the phosphorylation of STATs and play a critical role in signal transduction pathways associated with myeloid cell proliferation while JAK kinases mediate the activation of Erk-2 pathway which appears to provide antiapoptotic signals. Thus the activation of JAKs and STATs appear to be two independent but related events, which dictate two separate biological outcomes, the combination of which results in proliferation and survival of myeloid precursor cells.