Nucleic acid hybridization techniques are currently the most specific and sensitive procedures available for diagnosing and typing human papillomavirus (HPV) infection. HPV genomic DNA cloned into vector pBR322 or related vectors are commonly used as probes for detection of HPV. When isolating HPV insert DNA however, it is difficult to remove all pBR322 DNA. This can result in false positive readings when clinical specimens harbouring sequences homologous to pBR322 are screened. It was found that up to 200 ng of vector-like sequences in a clinical sample could be blocked by the addition of non-labelled, digested pBR322 sequences to the hybridization reaction.