The use of antigen fragments generated by specific proteolytic cleavage is a relatively simple "library" approach for epitope mapping in which possible overlapping fragments are screened with the antibody on Western blots. Proteolytic fragmentation with numerous proteases having different cleavage specificites can be carried out on native and denaturated proteins, generating a small and large number of fragments, respectively. To determine the antigenic site of a monoclonal antibody, we have examined the limited proteolytic digestion of the transducin alpha -subunit with four different proteases and detected antibody binding to fragments by Western blot. Using this approach, the epitope for this antibody was localized within the amino-terminal 17 residues of transducin alpha -subunit.