Placental protein 14 (PP14), also known as progestagen-dependent endometrial protein and pregnancy-associated endometrial alpha 2-globulin, is synthesized by the human secretory endometrium and decidua. We have isolated from a human decidual cDNA library clones corresponding to PP14 and deduced its entire amino acid sequence. PP14 contains 180 amino acids, 18 of which correspond to a putative signal peptide. The predicted molecular weight of the pre-PP14 is 20,555 and that of the mature protein is 18,787. PP14 is encoded by a 1-kilobase-pair mRNA that is expressed in human secretory endometrium and decidua but not in postmenopausal endometrium, placenta, liver, kidney, and adrenals. The 162-residue-long sequence of PP14 is highly homologous to beta-lactoglobulins, with a 53.4% identity with the amino acid sequence of horse beta-lactoglobulin I. The four cysteinyl residues (positions 66, 106, 119, and 160) responsible for intramolecular disulfide bridges in beta-lactoglobulins are all conserved in PP14. Southern blot analysis of human DNA suggested that PP14 gene sequences encompass some 20 kilobase pairs of the human genomic DNA.