The virions of all known viruses are composed of small amounts of genomic nucleic acid enveloped by proteins and other macromolecules. The aetiological agents of scrapie disease and the other subacute spongiform virus encephalopathies (SSVE), a group of slow, fatal degenerative diseases of the central nervous system, are, based on their resistance to sterilization and on indirect measurements suggesting subviral size, thought to have non-viral structures (see refs 1-3 for reviews). The kinetic studies reported here demonstrate that scrapie's resistance to many inactivants is limited to small subpopulations of the total infectivity, the majority population being highly sensitive to inactivation. Moreover, control inactivations of conventional viruses provide examples of both scrapie-like resistant subpopulations and complete insensitivity to virucidal agents, especially when those viruses, like scrapie, are suspended in hamster brain homogenate. Virus controls further establish that the ability of the scrapie agent to penetrate dilute agarose-acrylamide electrophoretic gels is shared by conventional viruses. Direct comparison of scrapie's resistance to ionizing radiation with the resistances of other viruses places scrapie with the smaller viruses, as opposed to requiring a subviral size as claimed.