A case of prolymphocytic leukaemia, showing several features not yet reported in this disease, is reported. The majority of lymphocytes in the peripheral blood and bone marrow had markers of both B- and T-lymphocytes. The simultaneous presence of receptors for sheep RBC and surface immunoglobulins on individual cells was demonstrated and the endogenous origin of these markers was established. The lymphocytes had some of the functional characteristics seen in chronic lymphocytic leukaemia (CCL). In vitro cell death in the presence of colchicine (colchicine ultrasensitivity) and polystyrene bead column retention were of the same order as seen in CLL. In contrast with the findings in CLL, these cells were markedly radioresistant in vitro. The dominant clinical features--anaemia and constitutional symptoms--appeared to be related to hypersplenism associated with massive splenomegaly. The relevance of these findings is discussed.