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One of the major successes of public health vaccination programmes has been the eradication of circulating poliovirus from the majority of countries in the world. Reintroduction of wild type poliovirus may occur following spread from sources of virus stored in virological and other laboratories for research purposes or from countries where polio remains endemic. It is therefore of obvious concern that the authors at the National Collection of Pathogenic Viruses (NCPV), whilst characterising stocks of rhinovirus from the MRC Common Cold Research Unit, found that five of 22 cell cultures had abnormal cytopathic effects when infected with putative rhinovirus. PCR and sequencing confirmed that four of these putative rhinovirus stocks (1A, 1B, 4 and 6) had similarities to polio type 1 sequences and were confirmed to contain poliovirus type 1 with likely contamination of two further serotypes. Genomic analysis of the poliovirus showed similarity to wild type virus, although the possibility that this was a vaccine strain that has undergone genetic drift remains. These findings should alert the medical and scientific communities to the risk of possible contamination of rhinovirus and, indeed, other virus stocks with poliovirus which has the ability to replicate in most cell lines and to overgrow more slowly replicating viruses. The authors advise that working stocks of virus should be checked regularly and eventually replaced with fresh virus from appropriate reference strains.