THE search for the relationship between structure and function of antibodies has gone through different phases. Much information has been accumulated concerning the interactions of antibody with antigens, particularly with respect to haptens of related and different structures. These data have given rise to a multitude of speculations concerning the chemical and physical structure of antibodies and the mechanism of antibody reactions. At present, major questions awaiting answers are: (1) Does antibody to a particular antigen have a unique primary structure ? (2) Do all antibodies have the same primary structure but they are folded differently ? (3) Are the antibody combining sites for a particular antigen the same, but are other parts of the antibody molecule modified for other functions ? (4) What is the composition and structure of the active site ?