Contemporary biochemists and molecular biologists strive to understand the relationship between the function and the detailed chemical structure of macromolecules. Whereas chemical and direct physical analyses are employed to probe molecular structure, specific antibodies to proteins have been invaluable reagents in the determination of the fine-structure of the antigen as well as the immunologic relationship of the antigen to other proteins. Often a crowning achievement to many years of arduous work purifying and characterizing a cellular protein is the production of specific antibodies to the protein. The availability of specific antibodies then propels the direction of research into investigations of entirely new areas of protein structure, function, and regulation that could not be approached by any other route.