Leukocytes from 33 patients with Crohn's disease, 20 patients with ulcerative colitis, and 20 patients with ankylosing spondylitis were tested for evidence of abnormal migration in the presence of preparations of colon from a patient with Crohn's disease. None of the patients was on treatment with immunosuppressive drugs. The test was also performed with leucocytes from 12 health subjects. Significant alteration was seen in the Crohn's disease group, particularly when the antigen used was mitochondrial or microsomal fraction of colon mucosa, whereas the patients in the ulcerative colitis group showed reactivity only with the whole colon homogenate and not with the subcellular fractions. The ankylosing spondylitis group showed no statistical difference from the normal controls with any of the antigens.