The diagnosis of acute undifferentiated leukaemia (AUL) is made when the cells of patients with acute leukaemias cannot be classified as myeloid or lymphoid by means of morphological, cytochemical and immunological criteria. The mononuclear cells of eight different AUL patients were cultured in suspension for 3 d with or without TPA. After culture, especially in the presence of TPA, the cells of all patients expressed at least one myeloid membrane antigen. It was shown that this antigen expression was dependent on de novo protein synthesis and not influenced by inhibition of proliferation.