A library of mutants of metastable proteins, such as proteinase inhibitors, can be screened for the specific loss of a wild-type capability to bind an antibody, yielding valuable drug-design information which otherwise is unavailable. By this approach, for example, a mutant proteinase inhibitor can be obtained that has the amino acid sequence of a wild-type protein, or an active fragment thereof, save for the presence of one or more mutations in at least one epitope, thereby altering interaction of the mutant with an anti-proteinase inhibitor antibody.