One of the major limitations of DNA flow cytometry (FCM) in hematologic malignancies is the lack of information about the proliferation activity of subpopulations of the heterogeneous bone marrow (BM) compartment. We studied the S-phase DNA content of immunophenotypically defined BM subpopulations (CD2+; CD19+; CD2/CD19+; glycophorin-A+; CD14+; CD13+; CD33+ and CD13/CD33+) in 18 patients with acute myeloid leukemia (AML), including three patients with M6 AML. The results were compared with the findings in twelve normal BM aspirates. The measurements were performed using a special protocol for bivariate FCM of DNA content and surface immunofluorescence (s-IF). In patients with AML the proportion of BM cells expressing the myelomonocytic and monocytic markers (M1-M5 AML) or erythroid marker (M6 AML) was expanded. However, in many patients other subpopulations were 4% or higher permitting the calculation of their S-phase DNA. No essential differences in median S-phase DNA percentages of the distinct subpopulations were observed between normal and leukemic bone marrow though the ranges in AML patients were much wider. These data suggest that AML is not characterized by an increased nor a decreased proliferation activity, but rather by a situation of cell growth independent to the normal regulatory mechanisms. Additional information was obtained upon DNA aneuploidy using CD2+ or CD2/CD19+ cells as an intrinsic DNA standard which allowed us to define differences in the DNA index as small as 2% as aneuploid. This approach appeared suitable for detecting small-degree numerical chromosomal aberrencies, as found by cytogenetics, in 4/6 cases.