The capacity of peripheral neutrophils for activation of the arachidonic acid (AA) metabolism was studied during phagocytosis of IgG containing immune complexes (ICs) binding to Fc-receptors. Release of approximately 9% of the intracellular pool of radiolabelled AA in phospholipids, and release of the pro-inflammatory mediators, leukotriene B4 (LTB4), constituting 1.8%, and 5-hydroxyeicosatetraenoic acid (5-HETE), constituting 2.9% of the total radioactivity released, were demonstrated in 15 patients with untreated Crohn's disease, 15 patients with ulcerative colitis, and in 15 healthy volunteers. The concentrations of LTB4 and 5-HETE released were within the range of chemotactic activity for the two lipoxygenase products. Multiple large IgG containing ICs were revealed in neutrophils after phagocytosis by immunofluorescence. A minor defect in the IC uptake in patients with Crohn's disease observed in the absence of complement only, did not result in a subnormal activation of arachidonic acid release or metabolism. The study suggests that complexes of the IgG-class previously demonstrated in chronic inflammatory bowel disease, particularly in Crohn's disease, may activate inflammatory neutrophils leading to release of significant amounts of the pro-inflammatory lipoxygenase metabolites, LTB4 and 5-HETE.