Lymphocyte subpopulations in peripheral blood were studied by the sheep red blood cell rosette technique for the identification of T‐lymphocytes, and immunofluorescence staining with rabbit anti‐human immunoglobulin sera for the identification of B‐lymphocytes. In Crohn's disease, the total number of lymphocytes was found to be reduced and an almost equal reduction in all the lymphocyte subpopulations studied was found. In ulcerative colitis neither the number of T‐lymphocytes nor of B‐lymphocytes were found to differ significantly from normal values and the same was true of all B‐lymphocyte subpopulations. However, the number of lymphocytes carrying neither surface immunoglobulin nor sheep red blood cell receptors was found to be significantly increased. The results were correlated to cl