Attempts were made to evaluate the possible role of neutral proteinases of polymorphonuclear leukocytes (PMNs) on intradialytic protein catabolism of patients undergoing regular hemodialysis therapy. Granulocyte elastase concentrations increased from 112.3 +/- 10.3 (0 hr) to 228.0 +/- 11.5 ng/ml (3 hr) in 33 patients, from 151.1 +/- 13.2 to 759.3 +/- 66.1 ng/ml in 37 patients and from 174.4 +/- 25.0 to 1203 +/- 102.8 ng/ml in 13 patients (normal value 60 ng/ml). Elastase-alpha 1PI values were significantly enhanced in diabetics, no differences were observed between patients with chronic glomerulonephritis, interstitial nephritis or polycystic kidney degeneration. Additionally, neutral proteinases of PMNs were followed up cytochemically in blood smears of the patients. Halo formation (ring-shaped area around each neutrophil due to erythrocyte degradation) was reduced immediately before and completely absent two hours after initiation of hemodialysis therapy due to an enhanced release of proteinases. Therefore, it is possible that neutral granulocyte proteinases participate in intradialytic catabolism.