Elevated numbers of peripheral T cells expressing the activation associated antigen T9 are found in patients with active Crohn's disease. Expression of T9 is found to be correlated to the activity of the disease. However the presence of activated peripheral T cells is not restricted to Crohn's disease, but could also be found in other maladies with a supposed involvement of the immune system, e.g. ulcerative colitis, sarcoidosis, connective tissue disease and after organ transplantation. Significant elevation of the number of activated T cells could not be detected in cases of viral or bacterial enteritis and coeliac disease. Analysing the subset of T9 positive T cells with regard to the expression of Fc alpha receptors, a significantly increased number of Fc alpha receptor positive cells, within the subset of T9 positive cells in the peripheral blood of patients with Crohn's disease and ulcerative colitis was found, which could not be demonstrated in the case of other diseases analysed in this study. Thus the T9+ Fc alpha receptor +T cell subset may be considered to be pathognomonic for inflammatory bowel diseases. Analysis of the regulatory properties of T9 positive cells, with regard to the immunoglobulin isotype secretion in a pokeweed mitogen stimulated autologeous B cell assay, suggests that peripheral T9 positive T cells are involved in the suppression of IgA synthesis or secretion.