The surface of the cells of the normal choroid plexus and of the plexus papillomas is coated by sialomucopolysaccharides, containing substances which are positively stained with the colloidal iron (Hale-) reaction. After pretreatment with neuraminidase sialic acid is removed rendering the membrane negative to the Hale reaction. Using FITC- or rhodamine-labelled PNA (Arachis hypogaea) and RCA (Rhicinus communis) lectins specific receptors are demonstrable. The identity of these distinct oligosaccharides containing receptors with the Thomsen-Friedenreich antigens suggests the possibility of an immunologic significance, e. g. in bacterial or viral infections of the brain. The application of histochemical techniques seem suitable to clarify the differential-diagnosis between choroid plexus papillomas on the one hand and metastasis of carcinoma and papillary ependymomas on the other hand.