Infectious prions containing the pathogenic conformer of the mammalian prion protein (PrP^Sc) can be produced de novo from a mixture of the normal conformer (PrP^C) with RNA and lipid molecules. Recent reconstitution studies indicate that nucleic acids are not required for the propagation of mouse prions in vitro, suggesting the existence of an alternative prion propagation cofactor in brain tissue. However, the identity and functional properties of this unique cofactor are unknown. Here, we show by purification and reconstitution that the molecule responsible for the nuclease-resistant cofactor activity in brain is endogenous phosphatidylethanolamine (PE). Synthetic PE alone facilitates conversion of purified recombinant (rec)PrP substrate into infectious recPrP^Sc molecules. Other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol, were unable to facilitate recPrP^Sc formation in the absence of RNA. PE facilitated the propagation of PrP^Sc molecules derived from all four different animal species tested including mouse, suggesting that unlike RNA, PE is a promiscuous cofactor for PrP^Sc formation in vitro. Phospholipase treatment abolished the ability of brain homogenate to reconstitute the propagation of both mouse and hamster PrP^Sc molecules. Our results identify a single endogenous cofactor able to facilitate the formation of prions from multiple species in the absence of nucleic acids or other polyanions.