The presence of SV40 DNA sequences in British cases of mesothelioma was investigated using previously reported PCR assays. Three different SV40 primer sets were used which gave positive detection rates of 100% (SV.for3/SV.rev), 42% (PYV.for/rev) and 15% (SV.for2/SV.rev). When 26 mesothelioma biopsies were tested. Negative samples included 22 semen samples from AIDS patients, 10 blood donor DNAs and the human mesothelioma cell line H-MESO-1. The problems encountered with these assays and the interpretation of the results are discussed.