217 sera from 33 patients with acute myeloid leukemia (AML), 42 with chronic myeloid leukemia (CML), 12 with acute lymphatic leukemia (ALL), 22 with lymphoma, and 20 with other malignant diseases were examined by a radioimmunological technique for the presence of antibodies against DNA. The levels of single-stranded DNA binding activity was significantly higher in all 3 types of leukemia and lymphoma compared to those of healthy controls. In contrast, none of these sera exhibited a positive reaction with double-stranded DNA. In some cases of leukemia, the level of serum anti-DNA antibodies increased after the decrease of the leukocytes count.