Scrapie is a slow infection affecting the central nervous system (CNS) of sheep and goats (Dickinson, 1976). Experimentally the agent has been passaged in mice, hamsters and a number of other laboratory animal species. Kuru and Creutzfeldt-Jakob disease (CJD) are related diseases occurring in humans (Gajdusek, 1977). These diseases are caused by agents that have unconventional characteristics compared to those of known animal viruses (Gajdusek, 1977). The unusual physical, chemical and biological properties of the scrapie agent have led to an abundance of theories concerning its nature. Each of the major classes of macromolecules has been suggested as an important component of the scrapie agent. Hypotheses have suggested that scrapie agent is composed exclusively of protein (Griffith, 1967), exclusively of polysaccharide (Gibbons & Hunter, 1967), or exclusively of nucleic acid (Diener, 1972). Other hypotheses have suggested that it is a replicating membrane component (Gibbons & Hunter, 1967) or a filterable virus (Eklund et al., 1963). This is but a partial list of the theoretical constructs that have been proposed.