The highly specific plasminogen activator inhibitor of placental type, PAI-2, occurs in the placenta in a low molecular mass form of 46.6 kDa, and in pregnancy plasma in a (possibly glycosylated) high molecular mass form of 60 kDa. Extensive knowledge is available about the functional properties of PAI-2 as a plasminogen activator inhibitor and about its molecular biology and regulation. Of the several placenta proteins (PP) isolated, one of them, PP10, has a molecular mass of 48 kDa and its occurrence in malignancy and in complications during pregnancy has been the topic of a number of studies, though its properties and physiological significance are unknown. The present findings constitute evidence of immunological identity between PP10 and PAI-2. The sections of the amino acid sequence of PP10 analysed here were found to have identical counterparts in the sequence of the low molecular mass form of PA1-2, but in several preparations PP10 was found to occur in an inactive two-chain form due to cleavage of an Arg-Thr bond, the two peptide chains being linked to each other by a disulphide bridge. The cleavage site is identical to that observed in the reaction between PAI-2 and urokinase. The results make it possible to coordinate and correlate the findings of many separate studies and our own observations on PP10 and PAI-2.