Human red blood cells can bind to human peripheral blood lymphocytes. The percentage of these rosette-forming cells (TEh) depends on various conditions including temperature and the use of selected foetal calf serum. It is improved by toluidine blue. Prior incubation at 37 degrees C is not necessary. ABO and rhesus antigen D have no influence on rosette formation. Analysis of human rosettes in T and B cell rich populations, in patients with chronic lymphatic leukaemia or T cell lymphoma provides further evidence that human autologous and allogeneic rosette-forming cells belong to a T cell subpopulation.