Lymphopoiesis was studied in 3-month-old normal C57Bl mice and in 3-month-old C57Bl mice carrying from 12 to 48 C57Bl thymus grafts using tritiated thymidine labeling. Thymus graft lymphopoiesis was found to be identical with that of normal thymus tissue and the presence of thymus grafts was found to have no influence on host thymus lymphopoiesis. No evidence was found that the massive amounts of thymus graft tissue in the mice affected any parameter of host lymph node lymphopoiesis nor was any evidence detected for the migration of thymic lymphocytes from these massive deposits of thymus graft tissue either to host lymph nodes and blood or to other organs in the host animal. It is concluded that the majority of small lymphocytes produced in the thymus and thymus graft tissue do not migrate from these tissues but die locally at the end of their intrathymic life span of 3 to 4 days.

Summary A cytological survey was made of thymic lymphoma tissue in AKR, (AKRxC3H)F1 and C3H mice with advanced spontaneous lymphoid leukaemia. In all strains, lyniphoma tissue contained a higher percentage of primitive lymphoid cells (large and medium lymphoma cells) than in the corresponding normal thymus. As a group, C3H and (AKRx C3H)F1 lymphomata were more highly differentiated than AKR lymphomata. Lymphomata in individual mice showed wide variations in the percentage distribution of the three types of lymphoma cells, but in any one animal the pattern of differentiation was the same in the thymus and lymph node lymphoma tissues. First generation thigh transplants of lymphoma tissue in AKR mice maintained the differentiation pattern of the original AKR thymic lymphoma tissue. It is concluded that in individual mice with lymphoid leukaemia a constant pattern of differentiation of the lymphoma tissue is maintained for the major portion of the duration of the disease.

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