Random peptide libraries (RPLs) displayed on the surface of filamentous bacteriophages have been extensively used as a tool to map epitopes or to identify antigenic mimics (mimotpoes) of disease-specific monoclonal antibodies or polyclonal sera. These RPLs are engineered by the insertion of degenerate oligonucleotides, encoding a specific number of random amino acids, in frame with a bacteriophage’s gene specifying a virion surface protein. The RPLs are constructed by inserting 21 to 36 random nucleotides into the gene for an appropriate coat protein in a phage which is propagated in E. coli. Particularly clear mimotopes have been obtained with monoclonal antibodies and with a few polyclonal antibodies against viruses. The choice and processing of sera for selection of disease related mimotopes and sera to remove mimotopes reacting with ubiquitous antibodies is important in studies of RPLs. We have used RPL to select mimotopes of antigens to be used for immunization against dermatophilosis. Immunoglobulin G (IgG) from sheep protected from dermatophilosis with an enzyme preparation from Dermatophilus congolensis, were used to select peptides displayed on phage in the Ph.D.-7 random peptide library which contains 109 peptides. Phage displaying peptides which were unrelated to the mimotopes associated with protection to dermatophilosis were removed with IgG from sheep which were vaccinated with the enzyme preparation but were not protected. The IgG from the protected sheep before vaccination was also used for negative selection. The selected mimotopes, those with clearly repeating motifs were chosen and used to immunize sheep. A mixture of four phage mimotopes induced antibody to a recombinant protease from D. congolensis. The immunized sheep, recovered more rapidly from the lesions caused by the strains when challenged with two strains of D. congolensis. Key words: Phage display, Random peptide library, Dermatophilosis, Serine protease

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