Flow cytometry of cellular DNA content revealed ploidy abnormalities in 15% of 170 patients with various leukemias, in 50% of 26 patients with malignant lymphomas, and in 68% of 110 patients with multiple myeloma, for an overall incidence of DNA content abnormality of 37% in 306 patients with hematologic malignancies. Since the incidence of ploidy abnormality in over 100 solid tumors exceeded 90%. DNA flow cytometry is also ideally suited to screen for bone marrow metastases. Cell separation by centrifugal elutriation was shown to permit enrichment of aneuploid cells, including one example where cells with abnormal DNA content were not recognized in the unfractionated sample. Biparametric measurements of acridine orange-stained cells for DNA and RNA content analysis were suitable to enhance the discriminatory power of flow cytometric detection of lymphoma and myeloma tumor cells in heterogeneous cell populations of bone marrow and lymph nodes. DNA/RNA measurements in leukemia, (the disease category with the lowest incidence of abnormal DNA mode) revealed a markedly higher mean RNA content in acute myeloid leukemia compared with normal or acute lymphoblastic leukemia bone marrow. While these different RNA content patterns were found in whole marrow, cell separation by centrifugal elutriation of normal marrow disclosed cell subpopulations of myeloid precursor cells with a RNA content pattern similar to that of unseparated AML marrow. Hence, the described differences in RNA content between normal and AML marrow seem to be related to the greater heterogeneity of differentiated cells in normal marrow and per se do not appear to be a unique feature of the leukemia disease process.