After eradication of circulating polioviruses, reintroduction might arise from cultured laboratory stocks, or from collections of patients\' or environmental samples. In an example of a further class of risk, we recorded poliovirus type 1 in at least four working stocks of what had been identified as various serotypes of human rhinovirus. This finding emphasises the need to urgently assess the possibility of presence of poliovirus, even in apparently well-characterised stocks of other viruses, to screen for the virus where necessary, and to destroy materials in which risk outweighs potential benefit.