Three aspects of immunological function were studied in patients with Crohn's disease and ulcerative colitis (inflammatory bowel disease): atopic status and serum IgE levels; serum concentration of C-reactive protein; and C3 activation. The incidence of atopy, assessed by prick testing with common allergens, did not differ in patients with inflammatory bowel disease from healthy controls. 12 of 39 patients with Crohn's disease and 5 of 20 with ulcerative colitis, among whom were some non-atopic subjects, had elevated serum levels of IgE. Serum levels of C-reactive protein in patients were significantly greater than normal, even in those in whom the disease was clinically quiescent. Symptomatic patients with Crohn's disease had significantly higher levels than similar patients with ulcerative colitis and in Crohn's disease the levels correlated well with an overall assessment of severity and disease activity. Although conversion of C3 was detected in fresh serum samples from inflammatory bowel disease patients and not controls, only minimal traces were present in just 7 of 89 samples of EDTA--plasma from 47 patients; this finding did not correlate with disease activity. However, there were low titres of immunoconglutinin in the sera of some patients, but not in controls, suggesting that complement activation may be occurring in vivo.