FRAGRAEUS was not able to show plasmocytes in normal thymus1 ; nor did she find plasmocytes in thymus after antigen injections2. After removal of the thymus in adult rats the antibody production is unchanged3. In spite of these results which ascribe no important influence on antibody production by the thymus there are still several clinical signs which lead to the conclusion that the thymus takes part in antibody production. Milton4 has noticed a strong barrier between thymus and blood circulation; he found that this barrier must be damaged so that antigens can reach thymocytes. After thymectomy of newborn 5–12 days old mice Milton found no germinal centres nor plasmocytes in lymph nodes. The purpose of this investigation was to establish: (1) if there are cell-fixed antibodies in thymocytes; (2) if there is a difference between an antibody stimulation with pure antigens and antigens mixed with Freund's adjuvant and alum precipitated antigens.