Tyrosine kinase Jak3 plays a critical role in the interleukin 2 IL-2 signaling because it not only participates the Jak-Stat pathway, but also interacts with unidentified signal transducers and regulates expression of some oncogenes such as c-fos and c-myc. Abundant evidence demonstrated that phosphorylated tyrosine was necessary for the interaction between two proteins. Therefore, in order to clarify the role of Jak3 in IL-2 signal transduction, the tyrosine-phosphorylation-involved yeast two-hybrid system was constructed and the N-terminal region JH3-JH7 of Jak3 was used as a bait to screen a peripheral blood cDNA library. About 50 double-positive colonies were obtained. Sequence analysis indicated that one of them was from nucleosome assembly protein 1 gene (Nap1), and encoded a protein of 392 amino acid residues. Two-hybrid system results demonstrated that interaction between Jak3 and Nap1 depended on the level of tyrosine phosphorylation. Furthermore, immunoprecipitation and Western blot experiments confirmed that Jak3 really interacted with Nap1 in murine pro-B lymphocyte BAF/BO3beta cells.