We have used intrathymic injection of fluorescein isothiocyanate to label thymocytes in situ. The method gives random labeling of the thymocyte population and so can be used to quantitate the extent of migration of cells from the thymus to the periphery. Migrant cells can be visualized in frozen sections or cell suspensions of peripheral organs by their fluorescence. Our data show that in young adults, about 1% of thymocytes leave the thymus per day. Since the bulk of thymocytes turn over every 5 to 7 days, this indicates that the vast majority (95%) of thymocytes die within the thymus. Cells that do leave the thymus, go mainly to the T areas of lymph nodes, spleen and Peyer's patches. Migrants are extremely rare in bone marrow, gut and liver. Migration is about the same in neonates as in adults relative to the size of the thymus, but is considerably lower in older animals where it is only about 0.1% of thymocytes per day at the age of six months.