Leukaemic clonogenic cells, capable of forming colonies of blast cells in an in-vitro assay, were examined for surface antigen expression using a panel of monoclonal antibodies (Mabs) to stem cell and myeloid differentiation antigens in nine cases of acute myeloid leukaemia (AML) and four cases of chronic myeloid leukaemia in myeloid blast crisis (CML-MBC). Clonogenic cells were found to be most frequently positive with anti-HLA-DR (positive in 100% cases) and RFB-1 (71%) Mabs, with significant reactivity also being seen with CD-33 (69%) and CD-13 (61%) myeloid specific antibodies. CD-11b and CD-15 antigens, expressed predominantly on mature leucocytes, were not significantly expressed on the clonogenic population. Interestingly, the CD-34 antigen, detected by MY-10 Mab on normal myeloid progenitor cells, was demonstrated on the clonogenic fraction of only one of seven cases tested. A discrepancy between antigen expression of clonogenic cells and immunophenotype of the total leukaemic population was frequently seen, with "early" markers (CD-33, HLA-DR, RFB-1) expressed on a higher proportion of the clonogenic fraction than the overall population, while the converse was the case for the "later" marker, CD-11b. Based on the known normal distribution of differentiation antigens, particularly the CD-13 antigen, cases could be ranked according to clonogenic phenotype into immature (CD-13- HLA-DR+ CD-33+ or CD-33-; five cases), and mature (CD-13+ HLA-DR+ CD-33+; eight cases), levels. However, there was no correlation between these maturation levels and the morphology according to the FAB classification. Of note, the mature group included three CML-MBC, as well as two AML cases with a history of myelodysplasia or myeloproliferative disorder. These immunophenotypic findings indicate a heterogeneity in the level of maturation of the clonogenic population, not only in cases of de-novo AML, but also in AML thought to derive from multipotential stem cells.