The determinations of the activity of lysozyme in the serum, lysosomes and lysosomal supernatant of neutrophil granulocytes were done in 33 patients with generalized psoriasis and in 19 healthy controls. A decrease of lysozyme activity was demonstrated in the serum, lysosomes and lysosomal supernatant in the patients. During treatment by the PUVA method this activity rose gradually above the values in the control group. Systematic determination of lysozyme activity may help in monitoring of treatment progress.