Activity of the kallikrein-kinin system and proteinase inhibitors were studied in blood plasma of children at various stages of tubulointerstitial nephritis, pyelonephritis and dysmetabolic nephropathy. Similar alterations in the activities of kallikrein-kinin system and in the rate of proteolysis inhibition were observed in tubulointerstitial nephritis and pyelonephritis. Supplementary demonstrations were obtained about development of inflammation in tubulointerstitial nephritis. Absence of the kallikrein-kinin system activation and a decrease in the alpha 2-macroglobulin content were detected in dysmetabolic nephropathy. Definite components of the kallikrein-kinin system and the proteolysis inhibitors may be used as indicators in differential diagnosis of tubulointerstitial nephritis and dysmetabolic nephropathy.