Male chickens of different ages were injected with tritiated thymidine into the parenchyma of the bursa of Fabricius. Using a radiochemical method based on DNA and tritium measurements, a transport of label from the bursa of Fabricius to the thymus, to the spleen, and to the caecal tonsils could be demonstrated in newly hatched, 9-day-old, and 6-week-old chickens, but not in 14-week-old chickens. Autoradiographic data showed that at least part of this transport was by bursa-derived cells. In the 6-week-old group, the localization of the migrating cells in the target organs was determined. Migrant cells in the thymus were found almost exclusively in the medulla. In the spleen, most of the cells homed within clusters of pyroninophilic cells in the red pulp. Bursa-derived cells could not be demonstrated in germinal centres with this technique, and significant cell traffic from the bursa of Fabricius to the bone marrow could not be detected. The pattern of cell migration correlates well with the morphological and functional maturation of the lymphoid system.