A rapid method for flow microfluorometric determination of DNA content per cell is described. Incubation of cells in a hypotonic solution of propidium iodide results in disruption of the cell membrane and rapid staining of nuclear chromatin. DNA distribution histograms generated from cells stained by this method are identical to those generated after fixation and RNase digestion. This technique is rapid (5 min of processing), requires minimal sample size, and avoids formation of cell clumps. It is suitable for clinical samples and tumor biopsies where sample size is limited. The method uses commercially available equipment and does not require special excitation wavelengths. Propidium iodide staining is more convenient and accurate than traditional methods involving fixation and RNase digestion. However, it may not be suitable for cells with fluorescent constituents or those with unusually large amounts of double-stranded RNA. The method is efficient and suitable for rapid analysis, making it ideal for clinical studies. The study was supported by a grant from the National Cancer Institute.A rapid method for flow microfluorometric determination of DNA content per cell is described. Incubation of cells in a hypotonic solution of propidium iodide results in disruption of the cell membrane and rapid staining of nuclear chromatin. DNA distribution histograms generated from cells stained by this method are identical to those generated after fixation and RNase digestion. This technique is rapid (5 min of processing), requires minimal sample size, and avoids formation of cell clumps. It is suitable for clinical samples and tumor biopsies where sample size is limited. The method uses commercially available equipment and does not require special excitation wavelengths. Propidium iodide staining is more convenient and accurate than traditional methods involving fixation and RNase digestion. However, it may not be suitable for cells with fluorescent constituents or those with unusually large amounts of double-stranded RNA. The method is efficient and suitable for rapid analysis, making it ideal for clinical studies. The study was supported by a grant from the National Cancer Institute.