HPV is a non-enveloped DNA virus comprised of an icosahedral capsid and circular, double-stranded DNA. HPV is classified into more than 50 distinct types based on base sequences. Anti-human papillomavirus, clone K1H8 (anti-HPV) reacts with a non-conformational internal linear epitope of a major capsid protein of HPV-1, which is broadly expressed among the different HPV subtypes. In Western blotting, anti-HPV identified a 57 kD protein, which corresponds to the molecular weight of a major polypeptide of the capsid protein VP1. Minor bands of 47, 44, 41 and 33 kD were weakly labelled and thought to be degraded capsid proteins. Anti-HPV has been demonstrated to identify HPV-1 antigen in ELISA and to detect HPV-1 and B6 by indirect immunofluorescence in infected frozen tissue sections. Anti-HPV was found to be immunoreactive with paraffin sections of formalin-fixed HPV-infected tissues which were demonstrated by Southern blot hybridization to be infected with HPV type 6, 11, 16, 18, 31, 33, 42, 51, 52, 56 and 58. Positive immunostaining was largely confined to the nuclei of infected cells. Occasionally, the cytoplasm of koilocytotic cells was observed to be immunoreactive.