In mouse, an oocyte-specific short isoform of DNA methyltransferase-1 (Dnmt1) lacking amino terminal 118 amino acid residues exists and plays a crucial role in maintaining the methylation state of imprinted genes during early embryogenesis [Howell et al. (2001) Cell 104, 829-838]. To address the question of whether or not Xenopus oocyte expresses such a short isoform, we raised monoclonal antibodies against the amino-terminal portion of Xenopus Dnmt1. Two of the isolated monoclonal antibodies, 3C6 and 4A8, were determined to recognize (1-32) and (115-126) of Xenopus Dnmt1, respectively. The amounts of Dnmt1 in Xenopus eggs were determined to be similar, 10.0 +/- 2.5, 8.0 +/- 0.8, and 8.2 +/- 0.2 ng per egg with monoclonal antibodies 3C6 and 4A8, and polyclonal antibodies, respectively. This indicated that Dnmt1 in Xenopus mature eggs had an identical amino-terminal sequence to the amino acid sequence deduced from the cDNA. Together with the fact that Dnmt1 in A6 cells immuno-reacted with all the monoclonal antibodies isolated and with the polyclonal antibodies, we concluded that Dnmt1 expressed in Xenopus mature eggs possesses an identical amino-terminal sequence to that in somatic cells. Immuno-purified Xenopus Dnmt1 in mature eggs showed similar specific activity to that in proliferating A6 cells and that of mouse recombinant Dnmt1.