alpha 1-Antichymotrypsin (ACT), which is known as an efficient serum protease inhibitor and is detected in tumor cell nuclei, was found to inhibit the activity of DNA polymerase alpha purified from human stomach adenocarcinoma. The concentration of ACT required for 50% inhibition was 1.0 mg/ml and the manner of its inhibition showed the partially competitive relationship between ACT and DNA in the assay system. Furthermore the removal of ACT by anti-ACT antibody lost its antichymotryptic and anti-DNA polymerase activities in parallel. On the other hand, it did not inhibit the activity of human DNA polymerase beta. Other human serum proteins including serum albumin, alpha 1-acid glycoprotein, alpha 1-antitrypsin, and immunoglobulin G as well as other protease inhibitors such as leupeptin, pepstatin, phenylmethylsulfonyl fluoride, and chymostatin did not affect the activity of DNA polymerase alpha. Furthermore ACT heated at 60 degrees C did not inhibit DNA polymerase alpha, although it could still bind to DNA as well as native ACT. It was therefore concluded that the inhibitory action of ACT on DNA polymerase alpha was a direct phenomenon unrelated to its protease inhibitory or DNA binding activities.