The region which codes for protein tyrosine kinase (PTK) of v-yes oncogene, a genomic segment of Avian Sarcoma Virus (ASV) Y73, was amplified using Polymerase Chain Reaction (PCR) and inserted into a Bam HI site of pGEX-2T vector. The catalytic PTK domain of v-yes was highly expressed as a GST-fused protein in E. coli. The 60 kDa fused protein was purified using glutathione-agarose beads, and the GST moiety was excised by thrombin treatment. The tyrosine kinase activity of the purified PTK was determined by an in vitro [${gamma}-^{32}P$]ATP phosphorylation assay employing, poly $(Glu,;Tyr)_{4:1}$ (PGT) as a substrate. The enzyme activity showed a requirement for $Mg^{2+}$ with an optimal concentration of 20 mM and high-substrate inhibition. The $K_m$ value for PGT was $2.6{mu}M$ and the $V_{max}$ value was approximately 34 nmol/min. Western blot analysis with the antiphosphotyrosine antibody demonstrated that the PTK (33 kDa) expressed in E. coli exists as an autophophorylated form with the phosphorylated tyrosine residue.