Immunological and biochemical markers of leukemia/lymphoma cells have provided valuable insight into hematopoietic malignancy and normal differentiation. The general assumption is that as early lymphoid cells become committed towards terminal differentiation they lose their capacity for bimodal differentiation and cells become restricted to B or T cell development and function. We have observed that phenotypically "late" leukemic B cells close to secretory stage can spontaneously express mature T cell antigens (T11, T4 and T8) after culture in vitro. In further studies of these cells, it was found that the biochemical marker lactate Dehydrogenase (LD) follows the intermediate pattern expressed by thymocytes rather than that of typical B cells. The expression of T cell antigens can be blocked by incubating these cells with the phorbol ester TPA (12-0-tetradecanoyl phorbol 13 acetate) which promotes unidirectional B cell maturation to plasmacytoid cells in a way that mimics normal B cell differentiation. These observations indicate that presecretory malignant B cells are still programmed to express T cell biochemical and antigenic markers and this expression can be influenced by environmental conditions in vitro.