Abstract Administration of neocarzinostatin (NCS) (10 μg/g) intravenously through the tail vein caused strand breaks in DNA of both resting and regenerating liver as measured by sedimentation in an alkaline sucrose gradient. However, such fragmentation could be largely prevented by perfusing before squashing the liver ; the inhibition appears to be more in the resting compared to regenerating liver. Furthermore, when NCS was given intraperitoneally (10 μg/g) either at 1 or 4 hr before killing, it did not cause strand breaks in liver DNA. Rat liver nuclear suspensions, which consist of intact nuclei with little cytoplasm, when incubated with NCS (2 μg/ml of incubation mixture), resulted in extensive strand breakage in liver DNA. These results suggest that NCS may not penetrate the resting liver cell, at least not rapidly, and that the observed DNA strand breaks may be largely due to the interaction of the circulating drug in the portal blood with DNA in vitro at one or more steps during the preparation and analysis of the liver cell DNA. Heat denaturation of NCS abolished the property of fragmentation of DNA.