The immunohistochemistry of the cell population of the human spleen was studied. As controls, a strictly defined group of five specimens were used. This study stresses the need for the utmost care in this selection procedure. Using enzyme- and immuno-histochemistry the lymphocyte subpopulations were studied as to their localization in the several specific compartments in the red and white pulp. The findings were compared to those in idiopathic thrombocytopenic purpura, autoimmune haemolytic anaemia and Hodgkin's disease. In the controls a consistent pattern was found. There were specific changes in each of the disease groups consisting mainly of shifts between the compartments.