The presence of circulating free DNA in the sera of cancer patients has been reported by different authors.Moreover, a hypothetical relationship between the DNA level and different stages of the disease has also been suggested.In this work, the free serum DNA level in various cancer patients was measured and consistent results with the reported data were found. (1,4) To assess the possible mechanism for the release of this fraction and to explain the high levels observed in cancer patients levels of the bivalent intracellularcation, magnesium were determined. Further to relate the process to the antioxidative mechanism of the cell membrane also selenium levels were measured simultaneously. The serum free DNA and the magnesium concentration of the patients before treatment were significally higher and the selenium levels lower than the control group. After appropriate the rapy the magnesium levels were found to decline while the high free DNA and low selenium values still persisted. These findings imply that the membrane stability and permeability is impaired due both to the neoplastic changes and later to the effect of chemotherapeutic agents with the result that material is released into the circulation. © 1991, Taylor & Francis Group, LLC. All rights reserved.