Lymphocyte subpopulations in thirty-nine patients with the tropical splenomegaly syndrome (TSS) were studied using the E-rosette, EAC rosette and direct immunofluorescent staining techniques. In the peripheral blood a decrease in the percentage of T-lymphocytes was found which was accompanied by an increase in the percentage and number of B-lymphocytes. These abnormalities reverted towards normal on treatment. Splenic aspirates contained an increased proportion of T-lymphocytes. The relative T-cell lymphoenia in TSS is probably due to specific trapping or non-specific sequestration of lymphocytes in the liver and splen. B-cell lymphocytosis is probably due to persistent antigenic or mitogenic stimulation by malaria.