Placental protein 12 (PP12) was originally isolated from term human placenta and adjacent membranes. Recently we found that the site of PP12 synthesis is decidua but not placenta. In this work, the purity of PP12 was first tested by sodium dodecyl sulfate polyacrylamide slab-gel electrophoresis and by reverse phase HPLC, and the N-terminal amino acid sequence of 15 residues was determined by a liquid-phase sequencer. A single amino acid sequence of Ala-Pro-Trp-Gln-Cys-Ala-Pro-Cys-Ser-Ala-Asp-Glu-Leu-Ala-Leu was obtained showing identity to the known N-terminal amino acid sequence of somatomedin-binding protein from human amniotic fluid. Like the latter, PP12 bound somatomedin (insulin-like growth factor I) as demonstrated in gel chromatography by a shift in the elution pattern of [125I]iodo-insulin-like growth factor I after incubation with PP12. These data show that PP12 is a somatomedin-binding protein and extend through previous literature on PP12 the existing knowledge on the physiology and pathophysiology of somatomedin-binding protein(s) in human reproduction and cancer.