SummaryReticuloendotheliosis virus (REV) proviral DNA appears to be a frequent contaminant in Marek's disease (MD) vaccines. A polymerase chain reaction (PCR) was established and evaluated for its ability to detect REV proviral DNA in infected cell cultures and chicken tissues. Deoxynucleotide primers were selected from the highly conserved gag region of the REV genome. The amplification products were identified by electrophoresis, nested PCR and by hybridization with a digoxigenine‐labelled oligonucleotide. The PCR results correlated well with the diagnosis obtained by conventional procedures, i.e. virus isolation or indirect immunofluorescence test (IIFT