The presence of circulating immune-complexes (IC) and their in vivo interaction with polymorphonuclear neutrophils (PMN) have been detected in two cases of idiopathic neutropenia with normocellular bone marrow. The injection of patients' sera into New Zealand White rabbits caused a striking neutropenia due to sequestration of PMN in the vascular bed of kidneys and lungs. The kinetics of PMN disappearance from peripheral blood and the pattern of sequestration overlapped that induced by the injection of pre-formed soluble IC. Treatment with plasmapheresis caused an early and lasting increase of PMN; rabbit PMN were almost unaffected by the injection of patient serum after the course of plasmapheresis. These data are consistent with the possibility that idiopathic neutropenia with normocellular bone marrow may be caused by persistent in vivo interaction between IC and circulating PMN.