Neutral proteinases of neutrophilic polymorphonuclear leukocytes were followed up cytochemically in blood smears of 12 patients submitted to regular hemodialysis treatment (RDT). Halo formation (ring-shaped area around each neutrophil due to protein degradation) was reduced in all patients with end-stage renal disease under RDT. Concomitant to the development of leukopenia, a maximal increase of the plasma levels of the granulocytic elastase in complex with alpha 1-proteinase inhibitor was observed 3 h after starting hemodialysis (+409%; p less than 0.001). On the other hand, the proteolytic activity of the plasma samples against azocasein as substrate, being significantly higher (+244%; p less than 0.001) in RDT patients compared with healthy controls, decreased permanently during therapy (-71%; p less than 0.001; 3 h after initiation of the treatment). The mechanisms of release as well as of elimination of proteolytic activity due to RDT are discussed.