In smear preparation of lymph nodes excised from 36 patients with a malignant lymphoma the DNA content was estimated in different tumor cells following Feulgen staining by means of quantitative cytophotometric measurements. The tumors investigated included 11 Hodgkin lymphomas and various types of Non-Hodgkin lymphomas (altogether 25). Whereas the small lymphoid cells found in Hodgkin lymphomas show a diploid DNA content with a wide scattering, in the large mononuclear reticular cells (Hodgkin cells) as well as in Sternberg's giant cells only aneuploid DNA values are seen lying in the hyperdiploid, hypertetraploid and hyperoctoploid regions. DNA stemlines are found in the aneuploid regions, but in some cases they are missing. This indicates that we are dealing with malignant tumor cells. In Non-Hodgkin lymphomas with a low grade of malignancy (centrocytic-centroblastic lymphoma, centrocytic lymphoma, CLL) an aneuploid DNA content is found particularly in the large cells (centroblasts, lymphoblasts) with or without a DNA stemline. These cells must be considered as primary malignant tumor cells. In the immunoblastic lymphoma of high malignancy only aneuploid tumor cells are present showing a particularly intense DNA content in the large immunoblasts, whereas a DNA stemline is usually missing. The range of aneuploid values, the number of polyploid tumor cells and the presence or absence of a DNA stemline are important criterions in determining the degree of malignancy. By means of cytophotometric measurements of the DNA content in the tumor cells of malignant lymphomas it is possible to assertain the degree of malignancy and to establish an objective prognosis.