We phenotyped 548 blast populations from patients with acute de novo leukemia with a panel of 27 monoclonal antibodies and an antiserum against the enzyme terminal deoxynucleotidyl-transferase (TdT). In 56 cases (10,2%) we found a marker profile consisting of both myeloid and lymphoid characteristics (biphenotypic) leukemia = interlineage infidelity). In 49 out of the 56 cases the blast populations were clearly related to the myeloid (n = 41) or the lymphatic (n = 8) system. In those blast populations only one characteristic of the other differentiation could be detected. However, in 7 cases more than one myeloid as well as lymphatic characteristics were found on the same blast cells. In 4 cases (0.7%) the immunological phenotyping indicated the presence of two separate blast populations. The occurrence of a stem cell leukemia, an abnormal derepression of a genome, a defect of the membrane synthesis and the existence of a normal counterpart are taken into consideration as a cause of an "atypical" marker profile.