Introduction of the maturation index (MI) as a measure for the degree of maturation improved the subtyping of acute myeloid leukemia (AML). A comparison is made here between the MI and the results of surface marker analysis with a panel of monoclonal antibodies (McAb) in the immunofluorescence technique. The McAb applied in 46 AML patients (greater than or equal to 15 years) were granulocyte specific (MI/N1, UJ 308, B4-3, B13-9), granulocyte-monocyte specific (OKM-1, B2-12) or had specificity for the Ia-like antigen (OKI-1), 'T-cells' (3A1), immature cells (OKT-10) or platelets (C17-28). In 32 of these patients more McAb could be investigated with specificities for granulocytes (VIM-D5), granulocytes-monocytes (RUPI-5), monocytes (MONO BRL, RUPI-4), erythrocytes (VIE-G4) and AML cells (VIM-S8). An increase in surface marker expression evident from the reaction with a number of McAb (UJ 308, B2-12, OKM-1 and OKI-1) paralleled the rise of the MI in FAB M5. A decrease of the expression of antigen detected by OKI-1 paralleled the rise of the MI in FAB M1-3. The granulocyte or monocyte specific McAb, as they are determined on normal human peripheral blood cells, did not distinguish between FAB M1-3 and M5. The maturation index seems to be a valuable tool in understanding the results of surface marker analysis.