<p>The possibility of using the immunoferment analysis for determination of lysozyme of mucus and fish tissues has been investigated. This method was adapted by reducing the dilution of the supernatant of biological material with buffer from 1:5 to 1:1. Clinically healthy fish (10 samples of pike and 10 samples of carp) were taken from fish ponds at water temperature of 14–16ºC. Biological material (blood, spleen, mucus of superficial integuments) was selected concerning the commonly used methods in ichthyopathology. The determination of lysozyme activity was carried out using high sensitivity RIDASCREEN^®FAST Lysozym test kit. When diluting the supernatant of biological material with buffer in a ratio of 1:5 (according to the method), it was not possible to calculate the data by the calibration curve, since the lysozyme parameters of the samples were lower than the lysozyme standard (0,050 mkg/g). Therefore, the final dilution was reduced and the experiment was continued with dilution of the supernatant with a protein extraction buffer in a ratio of 1:1. The data of the statistical processing of the results have led to conclusion the use of RIDASCREEN^®FAST Lysozym test kit in the proposed modification for the immunoferment analysis of lysozyme activity in tissues and mucus of fish is possible. A high content of lysozyme was noted in the mucus of the superficial integuments of fish, the lowest – in the blood serum. The content of lysozyme in the tissues and organs of pike is higher than in carp.</p>