Tyrosine phosphorylation is an early event of interleukin-11 (IL-11)-mediated signal transduction. The IL-11-responsive cell line 3T3-L1 (mouse preadipocytes) was tested for the presence of Src-family protein tyrosine kinases (PTKs). Only p62yes, p59fyn, and p60src were found to be expressed. Immune-complex kinase reactions using the artificial substrate enolase showed that upon stimulation with IL-11, p62yes and p60src were activated, but p59fyn did not appear to be activated. These results correlate with antiphosphotyrosine (APT) immunoblots. IL-11-stimulated cells also revealed increased phosphatidylinositol 3-kinase (PI3K) activity. Increased activity of PI3K was found to be associated with tyrosine phosphorylated proteins and p62yes, after IL-11 treatment. Immunoprecipitation studies with anti-PI3K revealed for the first time that p62yes associated with PI3K in response to IL-11. To summarize, IL-11 activates p60src, p62yes and PI3K but not p59fyn, and this activation of p62yes leads to its association with PI3K and PI3K activation.