Morphological evidence of activation in vivo of circulating neutrophils in patients with inflammatory bowel disease (IBD) was sought by quantitative light microscope examination of toluidine blue-stained preparations made from peripheral venous bloods that had been fixed immediately ex vivo. The proportion of spherical (unactivated) circulating neutrophils was reduced in active Crohn's disease (73%; 46-96 (median; range), n = 11) compared with inactive Crohn's (90%; 45-99; n = 18, P less than 0.01) and normal subjects (94%; 44-98; n = 13, P less than 0.05). There tended to be fewer spherical neutrophils in active ulcerative colitis (77%; 13-96; n = 17) than in quiescent colitis (88%; 57-99, n = 13, P less than 0.1) or normal subjects (P less than 0.05). Activated neutrophils occur in the circulating pool of patients with active IBD and can be detected by light microscopy of peripheral venous blood leucocyte preparations.