In the early phase of hemodialysis progressive decreases in some enzyme activities in the leukocyte homogenate, neutrophil granule fraction and postgranular supernatant were found with concomitant rise in plasma β-glucuronidase activity, which is indicative of the release of neutrophil granule factors into the extracellular environment. Intravenous infusion of human neutrophil granule products to rabbits induced profound transient neutropenia. The results suggest that the release of neutrophil granule factors in the early period of hemodialysis may be a possible cause of hemodialysis neutropenia