In order to study local immunoregulatory mechanisms in patients with seronegative spondylarthropathy and histological evidence of asymptomatic intestinal inflammation, we determined mononuclear cells in distal ileal mucosa with an indirect immunoperoxidase technique using a panel of monoclonal antibodies. The number of intraepithelial lymphocytes in inflamed mucosa was not significantly increased. They carried mainly the suppressor-cytotoxic CD8 marker (greater than 80%), less frequently the helper T cell marker CD4 and lacked the pan T epitope CD3 in greater than 50%. There were no intraepithelial B cells or natural killer cells. The lamina propria lymphocytes were T and B cells, the former being more numerous. Helper T cells were more frequent than suppressor-cytotoxic T cells. There were no alterations in the proportion of T cells in inflamed mucosa. Natural killer cells were scarce. They occurred in small numbers in a minority of cases with gut inflammation and spondylarthropathy and rarely in inflammatory bowel disease. It is concluded that although there is an increase of lamina propria monocytes, there is no immunoregulatory imbalance in the ileal mucosa of patients with seronegative spondylarthropathy.