Circulating immune complexes were studied in untreated Crohn's disease (CD) and ulcerative colitis (UC) by leucocyte phagocytosis. Neutrophils from normal donors took up large immunoglobulin-containing inclusions from 14 of 15 CD sera, 3 of 15 UC sera (p less than 0.002) and from none of 15 reference sera from healthy volunteers (p less than 0.002). In contrast, inclusions could not be demonstrated on direct microscopic investigation. Our study confirms the presence of circulating immune complexes in Crohn's disease. Predominance of IgG-containing complexes in this condition is consistent with a mucosal origin. Discrepant results obtained by direct examination and by incubation of sera from patients with normal test neutrophils suggest a defective immune complex phagocytosis in CD. In consistency with this possibility, control experiments revealed a markedly decreased complex uptake by neutrophils of CD patients in vitro.