A cell treatment solution and a method that is used for preparing a stained cell suspension that is provided to a measurement of nuclear DNA by flow cytometry. The cell treatment solution may include a surfactant, RNase, and a fluorescent dye. The surfactant may include, for example, a non-ionic surfactant, a zwitterionic surfactant, an anionic surfactant, and/or a cationic surfactant. In one method of the invention, stained cell suspension that is provided to a measurement of nuclear DNA by flow cytometry is prepared. The method may include adding a tissue sample to a cell treatment solution including a surfactant, RNase, and fluorescent dye, disaggregating the tissue sample, and filtering the disaggregated tissue sample. Another method of the invention includes disaggregating a tissue sample, preparing cell suspension by filtering the disaggregated tissue sample, and adding a cell treatment solution including a surfactant, RNase, and fluorescent dye.