Concentrations of extracellular DNA and RNA in the blood of healthy donors and patients with malignant and nonmalignant breast tumors were investigated. Cell-surface-bound extracellular DNA and RNA were detached by PBS-EDTA treatment or mild trypsin treatment of erythrocytes and leukocytes. In healthy donors, almost all extracellular nucleic acids (98%) are bound at the surface of blood cells. In the blood of cancer patients, extracellular nucleic acids were found in plasma and not at the cell surface. In patients with nonmalignant breast tumors, extracellular nucleic acids were found both at the surface of blood cells and in plasma. In healthy donors, the cell-surface-bound DNA is represented by 20-kbp DNA fragments and smaller fragments that varied in amounts in different fractions.