Acute leukaemia with mixed lymphoid and myeloid phenotype.

Three children with acute leukaemia had blasts that expressed both lymphoid and myeloid markers. The blasts met immunological criteria for acute lymphoblastic leukaemia (ALL)--common ALL antigen+, HLA-DR+, terminal deoxynucleotidyl transferase+--but their cytochemical features, including positive myeloperoxidase and Sudan black B, were those of acute nonlymphoblastic leukaemia (ANLL) as defined by the French-American-British Group. 30% of the blasts from one of two patients tested reacted with a monoclonal antibody specific for nonlymphoid cells (MCS-2). The wide overlap in the percentages of blasts expressing lymphoid or myeloid markers indicates that some leukaemic cells in each child had a mixed phenotype. There were no consistent cytogenetic findings, and the Philadelphia chromosome was not present. Complete remission was induced by treatment effective for either ALL (two patients) or ANLL. These three cases appear to represent a rare leukaemia subtype that we have designated acute leukaemia with mixed lymphoid and myeloid phenotype. Its recognition may be important in treatment, since two patients achieved remission with standard therapy for ALL. These cases demonstrate further the phenotypic heterogeneity that may be seen in leukaemic cell differentiation.