Aminopeptidase A, not yet defined aminopeptidases and endopeptidases, dipeptidyl peptidase I, II and IV, gamma-glutamyl transferase and oxytocinase were investigated in the normal human full-term placenta using qualitative (catalytic) cytochemistry, isoelectric focusing, immunocytochemistry and kinetic fluorometry. Aminopeptidase A could be visualized cytochemically in the smooth muscle cells of the chorionic plate, stem villi and basal plate blood vessels. Aminopeptidases were found in connective tissue fibres of the chorionic plate, villous stroma, basal plate and paraplacenta. Dipeptidyl peptidase IV was detected at the same sites as the aminopeptidases and, in addition, in amniotic epithelial cells, fibroblasts of the villous stroma, endothelium of chorionic plate and villous blood vessels as well as in the basophilic cytotrophoblast cells (x-cells) of the basal plate and paraplacenta, and it possibly also occurred in some domains of the plasma membrane of the syncytiotrophoblast and cytotrophoblast cells. The x-cells surrounded the fetus in the form of a dipeptidyl peptidase IV-positive shell at the border to the mother. The enzyme represented the first specific marker for x-cells. Dipeptidyl peptidase I and II were primarily found in Hofbauer cells (macrophages) of the villous stroma, but also in the syncytiotrophoblast, other villous stromal cells and cells of the chorionic and basal plate. gamma-Glutamyl transferase was present in some connective tissue elements of the chorionic plate. Oxytocinase and endopeptidases were not detected. Isoelectric focusing of proteases revealed different molecular forms of dipeptidyl peptidase IV in the paraplacenta and villous tree, while the aminopeptidases shared the same pattern in both regions.(ABSTRACT TRUNCATED AT 250 WORDS)