By autodigestion (endogenous endonucleolysis) of rat liver (RL) or rat-ascites hepatoma (AH) nuclei, the nucleosomes were released from the RL, but not from the AH, nuclei. In contrast, by micrococcal nuclease digestion (exogenous endonucleolysis), the nucleosomes were released more rapidly from the AH than from the RL nuclei. A 0.6 M NaCl extract of the RL or AH nuclei was filtered through a Sephadex G-100 column. The resulting topoisomerase fraction was subjected to DNA relaxation and catenation assays with pBR322 DNA as a substrate. Consequently, the relaxation activity was almost the same between the RL and AH fractions, whereas the catenation activity was much higher in the AH fraction.