The invention provides prion protein standards for use as reference materials for prion detection. The standard may be species specific, i.e. the standard is comprised of a preparation for detection of a single strain prion or it may be prepared to allow detection of multiple prion strains simultaneously. The invention also provides methods of preparing the prion protein standards using a group of non-human host mammals which have their genome manipulated with respect to genetic material related to a PrP gene such that the mammals are susceptible to infection with a prion which generally only infects an animal which is genetically diverse from the host.