Tec protein-tyrosine kinase (PTK) is the prototype of a new subfamily of non-receptor type PTKs, and is abundantly expressed in hematopoietic tissues. We have revealed that Tec is inducibly tyrosine-phosphorylated and activated by stimulation with a wide range of cytokines. To get more insight into the signaling mechanism through Tec, we have generated a constitutively active form of Tec PTK. Deletion of the Src homology (SH) 3 domain gave rise to a hyperphosphorylated and activated Tec kinase (Tec deltaSH3). The activity of Tec deltaSH3 was confirmed in 293 cells, as well as in cytokine-dependent hematopoietic cells (BA/F3). Tec deltaSH3 should be a useful tool to study the in vivo substrates of Tec PTK.