Circulating abnormal lymphoid cells in patients with lymphosarcoma cell leukemia show intense fluorescence when stained with fluoroscein-labeled anti-immunoglobulin. The fluorescence is brighter than normal B cells in contrast with chronic lymphocytic leukemia cells, which have much weaker fluorescence than normal. Also in striking contrast with chronic lymphocytic leukemia cells, lymphosarcoma cell leukemia cells redistribute the surface immunoglobulin rapidly during incubation at 37 degrees C to form bright caps. In this respect they more closely resemble normal B cells than chronic lymphocytic leukemia cells. In addition, in sequential studies, the appearance of these characteristics among the circulating cells of patients with lymphoma has been shown at a time when the appearance of a leukemic phase was equivocal morphologically. Thus these cellular characteristics may be helpful in the early diagnosis of a leukemic phase of lymphoma as well as distinguishing these cells from those in chronic lymphocytic leukemia.