Cell death releases products of chromatin catabolism, particularly DNA, which can be measured by a DNA assay (using the nick translation reaction) as a marker of toxicity. Conditions for blood and urine collection have been established for quanti-tation of extracellular DNA in mice. Toxic doses of lipopolysaccharide (5 mg/kg) and HgC2(3.2 mg/kg) release DNA to plasma and urine. Increase in urinary DNA is observed 24 hours after a single injection (5 mg/kg) of gentamicin. Quantitation of extracellular DNA can be used to investigate the occurrence of cell death in vivo and applied to toxicological studies, in animals and man.