Myeloid cell lines and control bone marrows demonstrate the reproducibility and accuracy of BCL-2 immunostaining. Cells were cultured, treated with therapeutic agents, fixed and immunostained with BCL-2–specific monclonal antibody and flouresceinated secondary antibody as described in Materials and Methods. Cells of the ML1 cell line showed consistently low BCL-2–positive staining fractions, while cells of the KG1a cell line showed consistently higher BCL-2–positive fractions (A). KG1a cells were used as interassay immunostaining standards and data presented in (A) include those collected for KG1a cells in flow cytometric analyses of all control bone marrow and AML sample preparations. Myeloid-enriched subpopulations were gated as described in the Results. Untreated cells in these subpopulations reproducibly demonstrated low BCL-2–positive fractions and increased positivity after treatments with therapeutic agents (B, C). BCL-2 – positive fractions were determined in comparison to fluorescence intensities of cells exposed to isotype-matched nonspecific antibody controls (B). Cells in lymphoid-enriched subpopulations of control bone marrows showed higher BCL-2-positive fractions (C).