This paper presents the findings of a basic study on biochemical characteristics and activity of lysozyme extracted from a commercially importantfish, Caspian kutum,Rutiluskutum,Kamensky1901. The enzyme was purified by the ammonium sulphate precipitation method and cationic exchange chromatography. Molecular weight of lysozyme was estimated as 15.8kDa by SDS-PAGE. The optimum pH and temperature were 6.0 and 45.0°C, respectively,whilethe Kmand Vmaxvalues were 0.00007g mL-1and 200units min-1, respectively. Thermo-stability of the enzyme was low at the temperatures higher than 50°C. The enzyme activity increased in the presence of NaCl (1-50mM), KCl (1-50mM) and CrCl2(1-5mM), then decreased at the higher concentrations. Furthermore,the enzyme activity significantly decreased in the presence of FeCl2and CuCl2(1-100mM), butnot stable in the presence of MgCl2. SDS inhibited the activity, but the enzyme exhibiteda good degree of resistance to urea. Fluorescence quenching was observed in the presence of FeCl2and CuCl2. Emission intensitiesin the presence of NaCl, KCl, CrCl2and SDS were elevated, while was negligible for the urea.Our results exhibitedthat NaCl and KCl are well-established saltsfor elevatingthe enzyme activity.However,the purified lysozyme did not displaya moderate stability against others salts.Besides, the purified lysozyme from R.kutumwas not heat stable.Therefore,some phenomena such as water hardness and warming can exert negative effects on the innate immunity of thefish. So,the enzyme structure can be made more stable by medium engineering through changing the salt composition of aqueous solution.

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