This paper is devoted to analysis of parameters of catalytical activity of CD45, the major transmembrane proteintyrosine phosphatase (PTP-ase) of the lymphocytes, isolated from plasma membranes of thymocytes of control and 0.5 Gy irradiated rats. CD45 catalytic features were evaluated using 0.2 mM sodium vanadate as the inhibitor and paranitrophenylphosphate (1-8 mM) and phosphotyrosine (1-6 mM) as, respectively, nonspecific and specific substrates. With the former, irradiation was shown to cause a decrease in Vmax but an increase in affinity. With phosphotyrosine both Vmax and affinity decreased. These data suggest that the exposure to radiation causes an increase in non-specific enzyme activity with a decrease in the ability to dephosphorylate the specific substrate. A study of cooperativity parameters shows that cooperativity between two phosphatase domains increased after irradiation. An analysis of the inhibitor kinetics showed that radiation caused a change of competitive inhibition by mixed one.