A case of acute leukaemia with Ph1-chromosome and monosomy 7 is reported, in which the peripheral blood contained three types of blast cell as distinguished by light and electron microscopy and immunological phenotyping. The first blast-cell type originated from the granulocytic lineage; the cells contained per-oxidase-positive granules, and had an HLA-DR+Tdt-CALLA-phenotype. The second blast-cell type was more difficult to define, but had many characteristics of the monocytic series. The phenotype of these blast cells was HLA-DR+Tdt-CALLA-BA-2+ or HLA-DR+/–TA-1+63D3+. Finally, the third type of blast cell was clearly of lymphocytic origin. These cells were peroxidase-negative, and were CALLA+ as studied by electron microscopy using immunogold labelling and fluorescence microscopy. Their phenotype was HLA-DR+Tdt+CALLA+. Cell sorting and double fluorescence assays showed that these three populations were separate; no cells of mixed myeloid/lymphoid phenotype were found.This case suggests the neoplastic transformation of an immature progenitor cell and subsequent differentiation of the neoplastic cells in various directions.