Proteolytic cleavage of antigen-monoclonal antibody (MAb) complexes can be a relatively simple and direct approach to identifying an epitope on a protein antigen. The antigen-binding domains of an antibody are resistant to proteolysis (1,2). They also confer protection against degradation of bound antigen, particularly in the immediate vicinity of the epitope (3,4). If an epitope is linear, it can be directly identified after its elution from the MAb following proteolysis of the antigen-MAb complex. Conformational epitopes can be localized, if not completely identified, by comparing the rates of peptide release from the free antigen and from the antigen-MAb complex. By analogy to DNase footprinting, a method to localize protein-binding sites on DNA by nuclease digestion of protein-bound DNA (5), proteolysis of antigen-MAb complexes to identify epitopes on protein antigens has been referred to as “protein footprinting” (6).