In order to understand further the molecular mode of action of 5-Aza-2'-deoxycytidine (5-AZA-dCyd), a potent antileukemic agent, we prepared enzymatically 5-Aza-2'-deoxycytidine 5'-triphosphate (5-AZA-dCTP) and performed studies with purified DNA polymerase alpha and DNA methylase from mammalian cells. DNA polymerase alpha catalyzed the incorporation of 5-AZA-dCTP into DNA. The apparent Km value for 5-AZA-dCTP was estimated to be 3.0 microM; the Km of dCTP was 2.0 microM. The apparent Vmax of 5-AZA-dCTP was slightly lower than that for dCTP. 5-AZA-dCTP was a weak competitive inhibitor (Ki 4.3 microM) with respect to dCTP. Template studies with 5-AZA-dCTP showed that this nucleotide analogue was incorporated into poly(dIC), but not into poly(dAT), suggesting that the incorporation follows the rules of Watson-Crick base pairing. Incorporation of 5-AZA-dCTP into hemimethylated DNA produced a significant inhibition of DNA methylase. These results show that 5-AZA-dCTP is a very good substrate for DNA polymerase alpha and that its incorporation into DNA inhibits DNA methylation.