Lymphocytes are the cells responsible for elaborating the recognition elements of the immune response; those molecules which make it possible to distinguish between different antigens. Even extremely similar antigens can be distinguished by this system; self can be discriminated from nonself; a huge array of viruses, bacteria, eukaryotic parasites and their products, and even closely related synthetic molecules can be recognized and bound specifically by these elements. It seems likely that over 106 different antigenic structures can be discriminated by the recognition elements capable of being produced by the lymphocytes of a single vertebrate animal. Furthermore, two distinct sets of lymphocytes, T cells and B cells, produce different types of recognition elements which are chemically dissimilar, which serve different physiological functions, and which are the products of entirely separate sets of genes.’ Those produced by B cells are ultimately secreted into the circulation and onto mucosal surfaces as immunoglobulin molecules, where they mediate a variety of effects after combination with antigen, whereas those produced by T cells remain associated with the synthesizing cell, where they function as receptors to trigger cell-mediated functions. The differentiation pathways which lead to the development of this capability for exquisitely specific antigen recognition and the means by which it is controlled are of considerable interest both to those who study the immune response itself and to those for whom it represents an accessible model for studying the fundamental mechanisms of cellular differentiation and gene control.