Sera from 103 patients with chronic inflammatory bowel disease (IBD) were tested prospectively for antibodies against neutrophil cytoplasmic antigens (anti-neutrophil cytoplasm antibodies, ANCA) and endothelial cell surface antigens (anti-endothelial cell antibodies, AECA) by indirect immunofluorescence (IIF) and assays based on whole fixed neutrophils, purified neutrophil enzyme substrates and human umbilical vein endothelial cells. Using IIF, ANCA were found in 26 IBD sera (25%) and in none of 51 controls. Twenty-two positive sera (85%) were from patients with ulcerative colitis (UC). The pattern of distribution of immunofluorescence was always perinuclear (P-ANCA). A majority of UC patients positive for these autoantibodies (68%) had active colitis, but none had evidence of vasculitis. Using a whole neutrophil ELISA, binding was demonstrable in 73% of UC sera compared to 27% of Crohn's (CD) sera and only 4% of controls. Unlike vasculitis sera, UC sera with P-ANCA did not bind strongly to myeloperoxidase (MPO). Forty-five per cent of IBD sera tested positive for IgG AECA in an endothelial cell ELISA, compared to seven of 51 (14%) controls. Binding correlated with both active and extensive colitis. A type of P-ANCA, in most cases distinct from MPO-specific P-ANCA observed in vasculitis, is detected in a significant proportion of patients with UC, but rarely Crohn's colitis and therefore may be of differential diagnostic value. IgG AECA are also frequent in CIBD sera but are less disease specific than ANCA.