The cDNA clones of two isoforms (enzymes I and II) of human placental ecto-ATP diphosphohydrolases have been isolated based on the N-terminal amino acid (aa) sequence of the immunopurified 82 kDa protein and characterized. The cDNA clone encoding enzyme I consists of 2081 nucleotides and the predicted enzyme I consists of 517 aa residues. Enzyme I has a 5'-UTR and an N-terminal 11 aa sequence that differ from CD39, but the rest of the sequence is the same as CD39. The hydropathy plot indicated that enzyme I has two hydrophobic regions near the N- and C-termini of the molecule. In contrast, enzyme II consists of 1814 nucleotides and the predicted protein consists of 306 aa residues. The sequence of 1-1018 nucleotides of enzyme II is identical to that of enzyme I, but the 1019-1814 nucleotide sequence is different from both enzyme I and CD39. The hydropathy plot indicated that enzyme II has one hydrophobic region near the N-terminus, suggesting that enzyme II is also anchored to the cell membrane. It is, however, likely that some of enzyme II exists as a soluble form in plasma, possibly after proteolytic processing.