AIM To study the activity of Ig-splitting proteinases of various classes in the urine in patients with chronic pyelonephritis. MATERIAL AND METHODS Clinical, device and laboratory examinations were made in 50 patients (age 20-60 years). Admission and dynamic measurements were also made of the activity of serin, metal-dependent and thiol Ig-splitting proteinases in the urine. RESULTS Total proteolytic activity in the urine of the patients was higher than in healthy subjects. Thiol proteinase activity was detected in 98% cases. This activity was 100 times higher in the patients than in healthy subjects. Significant differences were registered in the activity of serin proteinases. After the treatment the level of serin proteinases approached the normal value while thiol proteinase activity decreased significantly (2-fold, on the average) but remained high. If this activity was high, the patients (70%) had 1-2 exacerbations annually, in low activity exacerbations were absent. CONCLUSION The activity of Ig-splitting proteinases of different classes is an important diagnostic and prognostic laboratory criterion of chronic pyelonephritis and can serve a criterion of treatment efficacy.