Twenty-one cases of acute promyelocytic leukaemia (FAB M3) demonstrating t(15,17) chromosomal translocation were studied in detail by immunocytochemical techniques using a panel of monoclonal antibodies. A characteristic myeloid phenotype of the leukaemic cells, co-expression of CD9 and CD68 antigens and absence of HLA-DR, was noted in all cases. Although the cellular morphology of acute promyelocytic leukaemia provides the most rapid means for diagnosis, this unique leukaemic cell phenotype provides confirmatory diagnostic evidence. In view of the new therapeutic options and prognosis in acute promyelocytic leukaemia, the detection of cases with atypical morphology based on this unique phenotype would be of value. In addition, the phenotype of acute promyelocytic leukaemia is similar to that of basophils and mast cells and raises the possibility that the leukaemic cells may have undergone a degree of basophilic differentiation.