1. Three kinds of apurinic/apyrimidinic (AP) DNA endonucleases, APcI, APcII, APcIII were purified from rat liver chromatin. 2. Molecular weights of APcI, APcII and APcIII were 30,000, 42,000 and 13,000 Da, which have isoelectric points of 7.2, 6.3 and 6.2, respectively. 3. Mg2+ was essential for the activities of these 3 enzymes, and sulfhydryl compounds (beta-mercaptoethanol) had a stimulatory effect on the enzyme activities while N-ethylmaleimide and HgCl2 inhibited the enzyme activity. 4. Km values of APcI, APcII and APcIII for AP site of DNA were 0.53, 0.27 and 0.36 microM, respectively, and AMP was the most potent inhibitor to these three enzymes among nucleotides tested.