The idiotypic determinants have been precisely located to the variable regions of immunoglobulin polypeptide chains. Both chains are generally required to express the idiotype. The major idiotypic determinants are the result of the amino acid sequence of the hypervaiable regions, although some idiotypic determinants reside outside the antibody combining site and these so called "framework idiotypes" are important markers. In my view the hypervariable regions are spatially disposed so as to present adequate antigenic stimulation, and they display enough structural heterogeneity to account for the uniqueness of the idiotype in the general population of immunoglobulin molecules. Hypervariable regions, the antibody combining site, and the idiotypic determinants thus amalgamate three formerly diverse concepts into a unified theoretical construct.