In order to obviate some of the technical problems associated with preparation of monocellular cell suspensions required for flow fluorometry, isolation of nuclei from several types of benign and malignant human tissues was undertaken. Satisfactory preparations of nuclei were obtained from epithelia of the uterine cervix and colon and from lung tissue using the citric acid method. The sucrose method was effective with colonic epithelium only. Distribution of deoxyribonucleic acid content in these nuclei was measured based on green fluorescence of acridine orange and red fluorescence of propidium iodide in a Bio-Physics Cytofluorograph. Essentially diploid patterns of deoxyribonucleic acid distribution were observed for all benign samples regardless of tissue origin whereas the malignant samples gave histograms suggestive of abnormal deoxyribonucleic acid distribution. Preliminary observations on distribution of single-stranded nucleic acids using acridine orange red fluorescence showed marked differences between populations of benign and malignant nuclei. Isolated nuclei appear to be suitable for flow-through microfluorometric analysis and offer some significant advantages over intact cells.