Monoclonal antibody Ki-67 is a useful and easy means for evaluating cell proliferative activity. Furthermore, it appears to provide valuable prognostic informations in some neoplastic diseases. However, almost all studies on solid tumors were performed only by microscopic observation using immunohistochemistry, and not by flow cytometry. Therefore, the information deduced from the findings is limited. The method of DNA/Ki-67 double staining of solid tissue and cultured cells for flow cytometry is shown, and the significance and the usefulness of flow cytometric study of Ki-67 in clinical use are discussed.