With the increasing ability of modern medicine to transfer life-giving tissues and tissue products from one individual to another comes a parallel, often life-taking consequence of the unsuspected transfer of infectious passengers. Creutzfeldt-Jakob disease (CJD) has been transmitted by contaminated stereotactic EEG electrodes, neurosurgical instruments, dura mater allografts, and cadaveric pituitary hormone therapy. Widespread concern has been voiced about the possible risks of transmitting CJD through the administration of blood or plasma products, although no such case has so far been identified by epidemiological studies of at-risk populations. Recently completed experiments using hamsters and mice showed decreasing levels of infectivity in buffy coat, plasma, cryoprecipitate (the source of anti-haemophilic factor), and fraction I + II + III (the source of immune globulin). Preliminary results from further rodent experiments indicate that little or no infectivity is detectable in plasma during the pre-clinical phase of infection, that plasma infectivity is not reduced by either leukodepletion filtration or high speed centrifugation, and that about 5-10 times more infectivity is needed to transmit disease by the intravenous than intracerebral route. Altogether, the data from these rodent studies, especially when considered together with results from similar studies underway in primates, should provide a rational foundation for policy decisions by government agencies, plasma product manufacturers, and blood banking communities.