Lymphocyte development and ontogenetic changes in erythroid cells have been studied in chick-chick yolk-sac-embryo chimeras differing at the B locus antigens. Erythroid cells derived from the yolk sac or from the intraembryonic mesenchyme were demonstrated by indirect immunofluorescence in the peripheral blood of these allogenic chimeras. At 7 days of incubation, yolk-sac-derived red cells represent a majority in the peripheral blood. From 9 days of incubation onwards, embryo-derived erythrocytes appear in increasing proportions, making up approximately 90% of the peripheral blood cells at 17-18 days of development. After hatching, no yolk-sac-derived erythrocytes are found in the peripheral blood. Such a change from the yolk-sac-derived cells into embryo-derived cells was not observed in the lymphocytes, as analyzed using specific anti-B and anti-la antisera for detection of thymus and bursa cells, respectively. Ia-like antigens were detected on bursa cells using a triple layer immunofluorescence system. These results obtained from the allogeneic chimeras indicate that the early chicken yolk sac produces only transiently erythroid stem cells, while intraembryonic stem cells are involved in the production of definitive erythrocytes as well as of lymphocytes, both of T and B cells.