Chemotactic, phagocytic, and oxidative metabolic activity of exudative leukocytes was measured in patients with Crohn's disease (n = 20) and with ulcerative colitis (n = 20). Unstimulated and casein-stimulated migration in Boyden chambers did not differ from that of healthy controls (n = 21). Patients with Crohn's disease had reduced serum-independent phagocytosis compared with healthy controls (p less than 0.01) and patients with ulcerative colitis (p less than 0.01). Serum-dependent phagocytosis by leukocytes from patients with Crohn's disease did not differ from that in controls but was slightly increased in patients with ulcerative colitis (p less than 0.02). Unstimulated leukocytes showed increased oxidative metabolic activity in both patient groups compared with controls (p less than 0.01), which was negatively correlated with the disease activity in Crohn's disease (p less than 0.02). The study shows that mobilized leukocytes from patients with Crohn's disease differ from those mobilized in ulcerative colitis and supports the concept of an abnormal inflammatory reaction in Crohn's disease.