A method is described for three-color immunophenotyping and simultaneous DNA-quantification using a flow cytometer equipped with a 488-nm argon laser and a mercury lamp (UV). The approach includes reproducible immunophenotyping comparing antigen expression before and after cell manipulation for DNA-measurement. The coefficients of variation after DNA-staining (CV=3.13 for T-cells in peripheral blood and CV=3.38 for T-cells in bone marrow) were adequate for exact DNA-analysis. For aneuploidy detection, a true internal standard was established measuring, for example, the DNA-content of T-cells in B-cell disease simultaneously with the DNA-content of the malignant cells. Using this method, aneuploidies could be unequivocally detected in 17 out of 24 patients with multiple myeloma. Furthermore, intratumor heterogeneities in DNA-content and antigen expression could be recognized, allowing an exact separation of tumor cells and normal hematopoiesis. The study also demonstrated the importance of exact immunophenotypic characterization of lymphocyte subpopulations and the determination of their specific proliferation, for example after proliferation induction in cell cultures. Future studies should address the applicability of this rather simple multiparameter approach for simultaneous immunophenotyping and DNA-measurement especially in the detection of minimal amounts of aneuploid cells after chemotherapy.