Secreted forms of the sialoglycoprotein designated cellular prion protein (PrPC) have been identified that cannot be explained by alternative splicing. We report that secreted forms of PrPC derive from precursors that are bound to the plasma membrane by glycoinositol phospholipid (GPI) anchors. Secreted PrPC slowly appeared in the culture medium of metabolically radiolabelled cells after incubations of 8-24 h. Digestion of nascent PrPC with phosphatidylinositol-specific phospholipase C (PIPLC) prevented the appearance of secreted PrPC. Secreted PrPC partitioned into the aqueous phase of Triton X-114 like PrpC-released PrPC. While the M(r) of PIPLC-released PrPC was reduced 2-4 kDa after treatment with aqueous hydroflouric acid, which removes the entire GPI anchor modification, the M(r) of secreted PrPC was unchanged. Both PIPLC-released and secreted PrPC were recognized by antiserum raised against a synthetic C-terminal peptide corresponding to residues 220-233 (amino acid 231 is the site of GPI attachment). We conclude that GPI-anchored PrPC is post-translationally processed to remove most, if not all, of the GPI modification and then shed into culture medium. Whether PrPC is shed after proteolysis near the C-terminus remains to be established. A minority of PrPC in normal Syrian hamster brain partitioned into the aqueous phase of Triton X-114 like shed PrPC, suggesting physiological significance.