Thehck gene is member ofsrc family of non-receptor type tyrosine kinases. Here we report the nucleotide sequence of the rathck eDNA of 1.94 kb. The nuclcotide sequence shows an open reading frame coding for a polypeptide of 503 amino acids. A vector expressing a fusion protein of glutathione-S-transferase with 82 amino acids of the N-terminal region ofhck (from amino acids 32 to 113) was constructed, Using this bacterially expressed fusion protein antibodies were prepared which recognize the cellularhck gene product. These antibodies identified, by immunoblotting, two polypeptides of 56 and 59 kDa in rat spleen wherehck transcripts are present at high level. Immunoprecipitatedhck polypeptides were enzymatically active and were autophosphorylated in the presence of ATP and Mg2+. 1mmunoprecipitatedhck could phosphorylate exogenous substrates. Treatment of immunoprecipitatedhck by a purified protein tyrosine phosphatase decreased its enzymatic acitivity. Our results suggest that the enzymatic activity ofhck tyrosine kinase is regulated by phosphorylation and dephosphorylation.