Small fragments of the DNA of human papillomavirus type 16 (HPV-16) were randomly cloned into the bacteriophage fd which expresses the resulting peptides as part of its capsid. Antisera raised against different HPV-16 fusion proteins were used for screening of the phage clones and the reacting peptides were determined by sequencing the inserted HPV-16 DNA fragments of the positive recombinants. Seroreactive regions of the proteins derived from the E4, E6, E7 (two regions) and L1 (three regions) open reading frames could be found by this approach. Of these seven regions, four were defined by at least two overlapping inserts, thus limiting the domains to between 10 and 15 amino acids. In the case of the E4 open reading frame, the same region identified by immunoscreening was also found when synthetic overlapping octapeptides were tested by ELISA with the anti-E4 antiserum. Using an approach to predict 'receptor-like' regions within the respective proteins, five of the seven regions were also identified. From the data on these regions, synthetic peptides were produced and used for the detection of antibodies against HPV-16 proteins in human sera by ELISA.