In this paper is described by histochemical, polarization optical and electron microscopical techniques resp. an up to now unknown protein structure in the basal plasmoditrophoblast of the human placenta. This structure consists of noncollagenic filaments containing cystin or cystein. They are arranged in the plain in the form of a network and parallel to the surface of the chorionic villi. They occur in connection with Langhans cells, they regularly existed in young placentas, where they completely surround the chorionic villi, and they occur in the villi of termborn placentas only incomplete in parts of the plasmoditrophoblast. Differences of the molecular structure of the network of protein filaments, which are depended on the age, are not demonstrable.