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 degrees 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. (c) Human tumors in nude mice--breast cancer, lung cancer, melanoma and colon cancer. (d) Mouse ascites tumors--JB-1, L 1210, Ehrlich and P 383. It therefore seems well suited as a routine clinical procedure.