The percentage of lymphoid cells from chicken bursa of Fabricius, thymus, spleen, blood, cecal tonsils and Harder's glands reacting with an anti-chicken bursa cell serum (ABS) and an anti-chicken thymus cell serum (ATS) raised in turkeys was evaluated in a chronologic study from the day of hatching until 24 weeks of age using indirect immunofluorescence and lymphocytotoxicity tests. The predominant number of bursa cells reacted with ABS, while most thymus cells reacted with ATS. In these central lymphoid organs, the relationship between the two cell populations remained fairly constant over the whole observation period. In the spleen and-even more pronounced-in the lymphoid cell populations of blood, cells reacting with ATS were predominant. In contrast, in Harder's glands a majority of cells reacting with ABS could be detected from hatching until the 23rd week of age. Cecal tonsils contained equal amounts of cells reacting with ABS and ATS at 5 weeks, but a preponderance of ABS reactive cells at 10 weeks of age. These findings implicate very distinct relationships of lymphoid cell populations in different lymphoid organs of the chicken, which may account for the different time course in the acquisition of immunological competence in these various organs and are suggestive of their possible different responsibilities within the immunologic defense system.