Molecular cloning was used to express human papillomavirus type 6b (HPV-6b) antigens in Escherichia coli. Seven genomic DNA fragments of HPV-6b which together comprise the complete L1 and L2 open reading frames, known to code for capsid proteins, were cloned and expressed in E. coli as both beta-galactosidase and TrpE fusion proteins. Western blots of HPV-6b beta-galactosidase fusion proteins using 'genus-specific' antisera produced by immunization of rabbits with disrupted bovine papillomavirus type 1 (BPV-1) showed that polypeptides encoded by two DNA fragments from the mid portion of L1 of HPV-6b were cross-reactive. Only one of these two polypeptides reacted with antisera raised against disrupted HPV-1, directly demonstrating that this polypeptide contains the papillomavirus 'common antigen'. The cross-reactive region was confirmed by reversing antigen and antibody. Polyclonal antisera were raised against the seven HPV-6b beta-galactosidase fusion proteins and tested against BPV-1 virion proteins on Western blots. Only antiserum against the mid portion of L1 of HPV-6b reacted with the BPV-1 major capsid protein. HPV-6b fusion proteins were also used to test human sera for antibodies reactive in Western blots. Serum samples from 38 patients with documented HPV-6 infections and from 22 presumably uninfected controls were tested. Antibodies were not detected in any of the sera to any of the seven fusion proteins. HPV-6b beta-galactosidase fusion proteins are antigenic and can be used on Western blots to localize immunologically reactive sub-regions of proteins by reacting protein fragments with antisera from immunized animals. However, alternative methods will be required to detect anti-HPV antibodies in human sera.