Genes for two forms of human placenta diamine oxidase (dao) were cloned from a cDNA library and sequenced. One gene, pdao1, is identical in length to human kidney dao but differs from it by two bases in the coding region and differs slightly in the 3'- and 5'-noncoding regions. The second gene, pdao2, is nearly identical to these genes in the coding region, except that it has an extra 57-nucleotide coding segment near the 3' end of this region. This segment corresponds to the contiguous sequence of the 3' end of intron 3 of human kidney dao.pdao2 also differs significantly from pdao1 and human kidney dao in a 13-base sequence in the 5'-noncoding region. It is proposed that pdao1 and human kidney dao are polymorphic forms of the same allele. Whether pdao2 is a polymorph of these two is not certain, because of the significant differences in the coding and noncoding regions. Pdao2 may represent a different allele.