In mammals, the resetting of DNA methylation patterns in early embryos and germ cells is crucial for development. Two DNA methyltransferases, Dnmt3a and Dnmt3b, are responsible for the creation of DNA methylation patterns. Dnmt3L, a member of the Dnmt3 family, has been reported to be necessary for maternal methylation imprinting, possibly by interacting with Dnmt3a and/or Dnmt3b (Hata, K., Okano, M., Lei, H., and Li, E. ( 2002) Development 129, 1983 - 1993). In the present study, the effect of DNMT3L, a human homologue of Dnmt3L, on the DNA methylation activity of mouse Dnmt3a and Dnmt3b was examined in vitro. DNMT3L enhanced the DNA methylation activity of Dnmt3a and Dnmt3b about 1.5-3-fold in a dose-dependent manner but did not enhance the DNA methylation activity of Dnmt1. Although the extents of stimulation were different, a stimulatory effect on the DNA methylation activity was observed for all of the substrate DNA sequences examined, such as those of the maternally methylated SNRPN and Lit-1 imprinting genes, the paternally methylated H19 imprinting gene, the CpG island of the myoD gene, the 5 S ribosomal RNA gene, an artificial 28-bp DNA, poly(dG-dC)- poly(dG-dC), and poly(dI-dC)- poly(dI-dC). DNMT3L could not bind to DNA but could bind to Dnmt3a and Dnmt3b, indicating that the stimulatory effect of DNMT3L on the DNA methylation activity may not be due to the guiding of Dnmt3a and Dnmt3b to the targeting DNA sequence but may comprise a direct effect on their catalytic activity. The carboxyl-terminal half of DNMT3L was found to be responsible for the enhancement of the enzyme activity.