The IB-peptide marker in the V-region of mouse immunoglobulin light chains is genetically linked to the Ly-3.1 thymocyte surface antigen. The inbred strains of mice bearing these genetic markers demonstrate a high incidence of spontaneous leukemia. We are studying the relationships between these characteristics, and are emphasizing the determination of the structures of IB-positive immunoglobulin and the Ly-antigen. The 13-day foetal mouse thymus is being studied in organ culture to determine the biochemical steps involved in the differentiation and maturation of thymic lymphocytes. Emphasis is placed upon the role of the thymic epithelium in this process.