AbstractThe purpose of this study was to determine firstly whether the isolated enzyme DNA polymerase α, which functions within the DNA replicase system, exhibits different sensitivity against the thiol‐blocking agent 4‐hydroxy‐nonenal (HNE) when adult rat liver and the rapidly dividing Yoshida ascites hepatoma were used as enzyme sources and, secondly, whether the reaction catalysed by DNA polymerase is the most sensitive step of the DNA replicase system of native cells.DNA polymerase α as well as the non‐replicative DNA polymerase β, isolated from both sources, were remarkably similar with regard to their sensitivity against HNE, as indicated by the incorporation of radioactive label from [3H]deoxy‐thymidine‐triphosphate into DNA.The transport of [14C]thymidine through the plasma membrane and the incorporation of this precursor into DNA were studied with neonatal hepatocytes and with hepatoma cells. The incorporation of thymidine was inhibited at lower concentrations of HNE in both cell lines than the transport process and the reaction catalysed by DNA polymerase α. It was concluded that in the DNA replicase system of native liver and hepatoma cells another process different from the reaction catalysed by DNA polymerase α is more