The analysis of the examination findings of 54 patients with purulent pyelonephritis showed a great variety of immunological changes, which appeared both as differences in the types of abnormal immunity parameters and in the degree of the recorded pathological conditions. T-cell immunity deficiency was revealed in 55.6% of the patients, in a half of whom it was associated with humoral factor deficiency. In 25.9% of the patients the immunograms were similar to those in the controls and in 18.5% they were indicative of humoral immunity activation. Supplement of T-activin to the conventional antibacterial therapy contributed to an early immunological and clinical remission. The immunomodulatory effect was associated with the initial immune deficiency and manifested itself by higher relative and absolute T-lymphocyte contents along with elimination of their subpopulation imbalance. The effect proved to be less profound on the humoral immunity system. The arrest of an inflammatory reaction was evidenced by a decrease in beta-lysine levels in the serum up to normal ones. In contrast, antibacterial therapy without immunomodulation deteriorated the abnormal ratios of immunocompetent cells. The findings suggest that it is essential to take an individual approach to the use of immunomodulating therapy in patients with acute purulent pyelonephritis, taking into account not only clinical but immunological indications. It is most advisable to apply T-activin in cases of T-cell immune deficiencies with a significant imbalance of T-lymphocyte populations.