Observing the quantitative and qualitative composition of immunocompetent cell populations of the lymphoid tissue of the immunogenic organs allows to determine the immune status of the organism in a certain age period. The object of research is determing the cellular composition of the lymphoid tissue of the cecal Peyer’s patches and cecal (apical) diverticula in ducks in age-concerned aspect. Material simples for research were selected from broiler ducks of the Blagovarsky cross. Cytological testss were performed on imprint specimens. Immunoblasts, lymphocytes, proplasmocytes, and plasmocytes, monocytes, and macrophages are distinguished among the cells of the lymphoid tissue of the cecal Peyer’s patches and the cecal diverticula. The content of these cells is not the same. Population of lymphocytes in imprint specimens is the biggest. It consists of subpopulations of small, medium, and large lymphocytes, the ratio is uneven. The largest is a subpopulation of small lymphocytes, and the smallest is a subpopulation of large ones. The total content of lymphocyte in cecal Peyer’s patches and cecal diverticula decreases with age of the subject ducks. The content of small and medium-sized lymphocytes in the cecal diverticula and small lymphocytes in the cecal Peyer’s patches as well decreases. Simultaneously, the content of large lymphocytes in the cecal diverticula, large and medium lymphocytes in the cecal Peyer’s patches increases. The immunoblasts content in the lymphoid tissue of the studied immune formations decreases with age of ducks, while the quantity of macrophages and monocytes conversely increases. Proplasmocytes and plasmocytes are detected in the lymphoid tissue of cecal Peyer’s patches and cecal diverticula from the age of 10 days in ducks. Their content increases significantly with the poultry age. Reticular cells observation if complicated due to their location under a dense layer of lymphoid cells. Fibroblasts, M-cells, erythrocytes, and granulocytes in imprint specimens are detected in trace amounts. The established changes in the cellular composition of the lymphoid tissue of the cecal Peyer’s patches and the cecal diverticula in ducks in the age-related aspect confirm the occurring immune reactions within them. Consideration of these changes will improve the effectiveness of anti-epizootic measures
