We will describe a procedure to map epitopes on a protein against monoclonal IgGs. In this procedure, we amplified and mutagenized the entire or a part of the protein. Then, a DNA region encoding the protein was cut out by a restriction enzyme and ligated into a lambda-gt11 expression vector to construct a library. Thus, the protein is expressed as a fusion protein with beta-galactosidase. Protein in plaques obtained by phages derived from the library were tested for cross-reactivities by means of immunoblotting experiments.