In the peripheral blood (PBL) of juvenile chickens three populations of B cells have previouslv been distinguished based on life-span and origin of cells within each hopulation. In this revport we show that the largest PBL B cell subset, population 1 B cells, which are short-lived cells corresponding to about 60 % of PBL B cells and the vast majority of bursal emigrants, exit from the bursa directly from the follicular cortex. This conclusion is based on the specific labeling of rapidly dividing cortical lymphocytes with bromodeoxyuridine, followed by their detection in the periphery prior to the appearance of bromodeoxyuridine labeled cells in the bursa medulla. Furthermore, the rate of emigration of cortical lymphocytes, 1.00 k 0.1 '70 of PBL B cells per hour, is indistinguishable from the emigration rate of B cells from the bursa as a whole.The anatomical organization of the bursa has evolved to focus gut-derived antigens from the bursal lumen into the lymphoid follicles. The emigration of cortical bursal cells is discussed in relation to the exposure of bursal lymphocytes to extrinsic antigen. * Supported by the Medical Research Council of Canada Recipient of a Fonds pour la Formation de Chercheurs et 1'Aide a Senior Chercheur-Boursier of the Fonds de la Recherche en (MT10040) la Recherche Scholarship. Sante du Quebec.