Mulatero et al 1 report failure to detect Simian virus 40 (SV40) DNA in 12 British mesotheliomas. They propose that their negative results indicate that the previous positive findings are probably a consequence of PCR contamination. Since Mulatero et alsubmitted their paper, several laboratories have further confirmed the association of SV40 with human mesothelioma and other types of human tumours.2 ,3 Furthermore, the International Mesothelioma Interest Group has published results of their multilaboratory investigation confirming consistent association of SV40 DNA with mesotheliomas.4 Additional recent studies (reviewed by Butel and Lednicky2) have shown SV40 DNA to exist in an integrated form in human tumours and to be associated with expression of SV40 large T antigen as demonstrated by RNA in situ hybridisation, Western blotting, and immunostaining. These results therefore contradict the PCR …