An immune serum usually consists of a very heterogeneous population of immunoglobulins (Ig’s) the appearance of which in the serum has been induced by antigen. The antigen-ligating function of the Ig molecule (see Figure 1) is confined to the combining regions, which are two symmetrical areas at the solvent-exposed ends of the Fab arms of the Y-shaped Ig molecules. The combining region is situated in the variable (V-region) domain, a compact region consisting of the N-terminal half of the light (L) chain and the N-terminal quarter of the heavy (H) chain that is linked by sulfhydryl bonds. Between the areas of this domain occupied by the L- and H-chain V regions is a cleft exposed to the solvent. Antigens have been shown to bind in, or close to, this cleft (Amzel et al., 1974). An induced antibody population is said to be specific because it usually binds most strongly to the immunizing antigen and with lesser binding energies to certain compounds that resemble the immunogen in structure. Heteroclitic antibodies may be induced that bind more strongly to some determinant other than the immunogen (Mäkelä, 1965). In general, antibody populations show a high degree of specificity, in that they are able to discriminate among chemical compounds differing by as little as a single functional group, between stereoisomers, or between two proteins differing by as little as a single amino acid residue (Reichlin, 1974).