Cytophotometric DNA determinations on 26 cases of non-Hodgkin's lymphoma yielded the following findings: 1. Follicular centrocytic/centroblastic lymphomas (M. Brill-Symmers) and diffuse centrocytic lymphomas (lymphocytic lymphosarcoma) have a diploid DNA stem line. Diploid DNA values are observed in benign tumors, so that the assignment of these lymphomas to the group of "low grade malignancies" appears justified. 2. Lymphoblastic sarcomas show an aneuploid DNA stem line, as do 96% of all malignant tumors. 3. Lymphoid cells, plasma cells, and immunoblasts seen in immunocytomas are aneuploid. Thus these lymphomas must belong to the group of "high-grade malignant lymphomas" as regards their DNA distribution. 4. Immunoblastic sarcomas have aneuploid DNA stem lines (1 case tetraploid), in which both the lymphoid cells and the plasma cells from those immunoblastic sarcomas arising from immunocytomas show atypical DNTA distribution patterns. 5. In two cases of angioimmunoblastic lymphadenopathy, the lymphoid cells, plasma cells, and immunoblasts are aneuploid. They are thus regarded as "high grade malignancy" lymphomas. The results are discussed with respect to clinical course and prognosis. Measurements on a larger series of cases and correlation to clinical data are needed to support these results. Ultrafast DNA measurements made by flow-through cytophotometry can perhaps be helpful in the future for making the decision between a "low" or "high" grade malignant lymphoma.