Normal myeloid-enriched cell populations in control bone marrow samples show consistently low apoptosis responses to treatments with therapeutic agents. Primary bone marrow cells were cultured, treated with therapeutic agents, and prepared for flow cytometric analysis as described in Materials and Methods. Immature myeloid cells and monocytes were first recognized and flow-sorted from unfixed, CD45-immunostained bone marrow samples and then ethanol-fixed and stained with 7AAD, a DNA-specific stain. This demonstrated that the same myeloid versus lymphoid enrichments could be obtained by software gating DNA × SSC plots. Doublet discrimination gates were also applied to all flow cytometric analyses of fixed cells. (A) Representative histograms from flow cytometric sorting and follow-up cell-cycle/apoptosis assays of untreated or gamma irradiated cells from one control bone marrow sample. Sub-G1 apoptotic cells in lymphoid and myeloid subpopulations of an irradiated aliquot of this sample are noted. Cell-cycle distributions and apoptosis frequencies were determined by curve fitting using the Multicycle AV DNA analysis program. (B) Sub-G1 apoptosis data from this assay and its replicate and from assays of each of five other, independent bone marrow samples means and SEMs denoted for each treatment condition. Apoptosis frequencies are expressed relative to the cell fraction in G1 + S + G2/M compartments.