Bone marrow of chickens of different ages was labelled locally in vivo with small amounts of 3H‐thymidine directly into the bone marrow. Using autoradiography and a radiochemical technique, bone marrow‐derived cells were demonstrated in the thymic medulla, the bursa of Fabricius, the splenic red pulp, and the cecal tonsil and in the bone marrow from locations other than the labelled ones. Considerable numbers of bone marrow cells were found in the bursa of chickens labelled on the day of hatching, but hardly ever in older animals. Therefore, these cells probably demonstrate traffic of progenitor cells for the bursal cell line. No bone marrow‐derived cells could be localized in the germinal centers of the spleens or the cecal tonsils.

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.

Male chickens of different ages were injected with tritiated thymidine into the parenchyma of the thymus and the transport of thymic label and of thymus-derived cells to other lymphoid organs was studied. Using a radiochemical method based on DNA and tritium measurements in different organs, a quantitatively significant transport of label from the thymus to the spleen, the caecal tonsils, and to the bone marrow could be demonstrated in the 9-day-old chicken. The transport to the spleen was shown to continue in the 6-week-old and 14-week-old animal. Autoradiographically, thymus-derived cells could be found in the spleen and caecal tonsils as early as in the newly hatched period. Autoradiographic data also indicated that the traffic increased up to 6 weeks of age but decreased in older animals. In the 6-week-old group, the localization of the thymus-derived cells in the target organs was determined. Such cells in the spleen were distributed almost equally between the red and white pulp. Very few of the thymus-derived cells were highly pyroninophilic. None could be found in the germinal centres. No sign of cell transport from the thymus to the bursa of Fabricius could be detected. The pattern of cell migration is well correlated to the morphological and functional maturation of the lymphoid system.

Mouse thymus-processed lymphocytes (T cells) sensitized against histocompatibility antigens can be specifically adsorbed on, and eluted from, fibroblast monolayers bearing these antigens. Not only normal, but also glutaraldehyde-fixed fibroblast monolayers can be used for specific adsorption. This shows that glutaraldehyde-fixed histocompatibility antigens can be recognized by specific receptors on T cells.

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