To determine whether patients with Crohn's disease have a defect in immune regulation, suppressor T cell activity was assessed in 16 patients with mild or inactive Crohn's disease and was compared with that of an equal number of randomly selected normal controls. Pokeweed mitogen-stimulated cultures of peripheral blood lymphocytes from patients synthesized as much IgM as those from normal controls. In addition, cocultures of patient and normal peripheral blood lymphocytes did not result in either suppression or enhancement of immunoglobulin M synthesis. In contrast to these results with cultures of peripheral blood lymphocytes, cultures containing optimal ratios of purified B cells and T cells from patients synthesized significantly less immunoglobulin M (p < 0.005) than those from normals; in fact, the latter cultures from 6 patients synthesized no detectable immunoglobulin M. Detailed studies of cells from these patients indicated that a suppressor T cell was revealed in vitro during the cell-purification procedure. Finally, in those patients in whom it could be measured, radiation-sensitive suppressor T cell activity was found to be normal. We conclude that there is no deficiency of suppressor T cells regulating antibody synthesis in patients with Crohn's disease; on the contrary, at least one-half of these patients have suppressor T cells that markedly inhibit the synthesis of immunoglobulin M, but which are revealed only after purification of the T cells. © 1981.