During hemodialysis of heparinized blood without having a patient in the circuit, the serum concentrations of lactoferrin, myeloperoxidase (MPO) and ëosinophil cationic protein (ECP) steadily increased, indicating neutrophil and ëosinophil degranulation. The increments in serum of these granular proteins were more pronounced using plate dialyzers than capillary dialyzers. The release of granule constituents does not seem to reflect merely a sequestration of granulocytes in the dialyzer, since the increase of the serum concentrations of lactic dehydrogenase was very modest. The intracellular contents of lactoferrin, MPO, lysozyme and ECP were reduced after experimental dialysis in the granulocytes isolated from the blood, indicating that the cells in association with degranulation were not entrapped in the dialyzer. The relatively modest increase of the plasma concentrations of lysozyme during experimental hemodialysis, in spite of the reduction of the intracellular content of lysozyme, was explained by the propensity of lysozyme for adhering to the dialysis membrane. Serum samples obtained at different times during dialysis did not induce an enhanced release of granular proteins from isolated granulocytes in vitro. The earlier observed increase during hemodialysis of the serum concentrations of granular proteins in uremic patients can be explained by the dialysis membrane triggering granulocytes to degranulate.