The structure, development, lymphoid cell types, and capacity for antigen uptake of bronchus-associated lymphoid tissue (BALT) in chickens and turkeys were studied using light and electron microscopy, immunocytochemistry, and image analysis. The structure of avian BALT was very similar to mucosa-associated lymphoid tissues described in other species and tissue sites. Individual BALT nodules were composed of a population of lymphocytes covered by a modified epithelium. Age-related changes in BALT structure were observed, which may influence epithelial permeability to antigens and the potential for interaction between epithelial cells and lymphocytes;The number of IgA-, IgM-, and IgG-producing cells per unit area of BALT tissue was determined in 1-day, 1-, 2-, 3-, 4-, 6-, and 8-week old chickens. Comparisons were made among immunoglobulin isotypes, ages, and two tissue regions (BALT-associated mucosa and mucosa not infiltrated with lymphocytes). There were no immunogobulin-producing cells in 1-day old chickens, IgG+ cells only in 1-week old chickens, and cells of all three isotypes in chickens 2 weeks of age and older;Monoclonal antibodies were used to label T lymphocyte subpopulations (CD4 and CD8) and B lymphocytes (CB3) in chicken BALT. In general, CD4+ cells formed tight clusters capping germinal centers and were associated with blood vessels in BALT. CD8+ cells were diffusely scattered throughout lymphoid nodules, extending up to and into the epithelium. CB3+ cells predominated in germinal centers and overlapped CD4+ clusters around germinal centers;To monitor ability of BALT epithelium to take up antigens, three tracers (ferritin, live Bordetella avium, and ultraviolet-killed B. avium) were instilled into tracheas of 3-week old turkeys. Ferritin was found in both ciliated and non-ciliated epithelial cells and in basal extracellular spaces. Both killed and live B. avium were taken up by ciliated and non-ciliated cells of the BALT epithelium, and by intraepithelial macrophages;The results of these studies support a role for BALT in initiation of respiratory humoral immune responses in chickens and turkeys.