The percentage of lymphoid cells carrying immunoglobulin surface determinants was examined for the yolk sac, bursa of Fabricius, thymus, bone marrow, blood lymphocytes, spleen, cecal tonsils and Harder’s glands in normal White Leghorn chickens from the 11th day of incubation until the 26th week after hatching, using direct immunofluorescent staining with a fluoresceinated rabbit anti-chicken immunoglobulin serum. The results clearly show that under the test conditions employed surface immunoglobulins are markers of a bursa cell population or of bursa-derived cells, whereas thymus cells react only minimally with anti-immunoglobulin conjugates in young chickens. The migration pattern and maturation of bursa cells were studied chronologically as reflected in appearance, quantity and morphology of immunoglobulin surface determinant bearing cells in different organs.