Production of immunoglobulins (G, A, M) by pokeweed mitogen stimulated peripheral blood lymphocytes was studied in 81 patients with inflammatory bowel disease and compared with 40 patients with mild gastrointestinal disorders (controls). Immunoglobulin production was dependent on the concentration of mononuclear cells in culture, being maximal at the lowest concentration used (2.5 X 10(5)/ml). Adherent monocytes exerted suppression when cultures were reconstituted with more than 20% of these cells. T lymphocyte depleted cells (B cells) demonstrated T cell helper/suppressor dependence for immunoglobulin production in an isotype specific manner, the optimal T cell concentration for 'helper' activity being lowest for IgG and highest for IgM. In patients with active ulcerative colitis (UC) there was a reduction in the T cell concentration for optimal helper activity that was not isotype specific suggesting an increase in non-specific T cell helper activity. T cell helper activity reverted toward control levels in patients with UC in disease remission, except in the case of IgA production where there was a significant diminution of IgA production and of T helper activity for IgA synthesis. Patients with Crohn's disease were distinguished from both UC and control patients by: (1) reduced immunoglobulin production at low lymphocyte concentrations; (2) reduced monocyte-mediated suppression of immunoglobulin production and (3) no shift in T cell concentration for optimal helper activity for IgG and IgA with active disease.