A new modification of our detergent technique for the preparation of nuclei for flow cytometric DNA analysis is described. The attainment of low coefficients of variation of the peaks and of quantitative staining of nuclei from different tissues was a problem with the original method. This was solved in the new modification by trypsinization of the unfixed nuclei. The nuclei were stabilized by spermine. A simple procedure for long-term storage of samples at -80°C was integrated into the method. The fluorescence of the nuclei was stable for at least 3 hours after staining. Light exposure protection of the samples was essential. No cell loss was caused by storage or staining. The method was successfully applied on samples including: (a) Normal tissues-human lymphocytes, granulocytes and spleen. Mouse lymphocytes, bone marrow, spleen, liver, kidney and thymus. (b) Human neoplasms-lung cancer, breast cancer, lymphoma, leukemia, bladder cancer and cancer of the oral cavity. Key terms: Flow cytometry, DNA, propidium iodide, solid tissues, preparation, storage A previously published method for the preparation of fineneedle aspirates from solid tissues for flow cytometric DNA analysis (9) has been useful for examination of human solid tumors (1,13,14). The method is based on detergent lysis of cell membranes and produces stained nuclei in monodisperse suspension in a single step. However, the low salt procedure of the technique, which has been used most extensively, has two major drawbacks: (a) Tissues rich in cytoplasm, for instance liver cells, are not prepared optimally because residual cytoplasm contaminates the nuclei, causing clumping and nonspecific staining. In the DNA distribution this produces asymmetrical peaks with high coefficients of variation (cv). ( b ) Staining of different types of cells is not perfectly quantitative. Thus a comparison of human lymphocytes and granulocytes for example showed a 6% difference in fluorescence.The original method was therefore modified with the aim of obtaining a preparation technique that would produce optimal results in a broad spectrum of tissues in regard to