DNA-estimations, done by flow-through-cytophotometry, were performed in 18 malignant tumors of testis. Aneuploid stemlines which are specific for cancers were found in 17 cases. The only exception was a so called "occult seminoma". It showed only a small difference between the stem-line of tumor and normal cells of testis. So, by using DNA-estimation, it was possible to make a correct diagnosis all 18 patients, suffering from tumor of the testis. The aneuploid stem-lines of seminomas did not differ from those of other cancers. The type of proliferative behavior seems to be a criterion for prognosis. Lymphatic nodes with and without formation of metastases very clearly showed differences in the DNA-content of their nuclei.