Prion diseases of humans and animals are associated with the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(C)). Transmission of these diseases between mammalian species is usually limited by a 'species barrier', which can be mediated by differences in primary sequence of the prion protein between donor and host species. Studies on species barriers usually rely on the development of clinical disease in inoculated animals as an assessment of susceptibility in a particular host. Recent studies by a number of groups have demonstrated that the absence of clinical symptoms does not necessarily exclude transmission of prion disease across a species barrier. Such results indicate that subclinical or carrier states exist in these diseases, which has public health implications regarding human exposure to BSE prions and iatrogenic transmission from apparently healthy humans. Here the issue of subclinical prion diseases is reviewed and implications are discussed.