AbstractMature T cells found in the lymph nodes and spleen have the capacity to become activated and to proliferate in response to foreign antigens. The response of the thymus to such immunization is less well understood. We have examined one aspect of the thymic response by determining the effect of peripheral immunization upon cell emigration from the thymus. BALB/c (Mls‐1b) mice were injected with spleen cells from DBA/2 (Mls‐13) mice, and Vβ6+(Mls‐13‐reactive) thymic emigrants were identified 3–30 days after immunization. Neither the rate of total cell migration from the thymus nor the proportion of Vβ6+cells was altered, even though the immunizing spleen cells elicited an immune response in the draining (parathymic) lymph nodes. The same immunogen caused deletion of Vβ6+cells in both the thymus and lymph nodes after intraperitoneal injection into the neonate. The inability of DBA/2 splenocytes to modify the development of adult thymocytes after intrathymic injection of the cells precluded the lack of entry into the thymus as the reason for the lack of any observed effect in the adult. Our results, therefore, indicate that the development of adult thymocytes is not modified by immunization, and suggest that the differing thymic response of mice injected as adults or neonates is related to changes in the intrathymic antigen presentation capacity associa