Eighteen 2-month-old guinea fowl (Numididae meleagris) birds were intravenously infected with 100 L H6N2 virus, while three formed a control group -non-infected. The birds were clinically examined daily throughout the entire experimental period and no clinical symptoms of disease were observed. On days 7, 14 and 21, six infected and 1 control birds were slaughtered for pathological investigations. All visceral organs were macroscopically analysed and samples from lungs, heart, spleen, liver, kidneys, pancreas, thymus, bursa and duodenum were immediately removed and fixed in 10% buffered formalin for at least 2 days. Slices of 5 μm thickness were prepared, embedded in paraffin and stained with haematoxylin and eosin (H/Е) by standard procedures. The preparations were examined on Leitz light microscope. From the conducted pathoanatomical examinations, notable findings included the smaller size of the spleen, thymus, and the bursa of Fabricius in all examined birds, compared to the control group. Microscopically, however, as a constant find in all infected birds, we observed reactions of different type and extent within the lymphoid tissue of the central and peripheral immunocompetent organs, which could be summarised in two primary groups -lymphoid-proliferative and degenerative. The changes observed in the birds' euthanised 21 days p.i. were considerably more pronounced. Within the organs of the central immune system (thymus and spleen) lesions of the atrophic-degenerative type were found out. The organs of the peripheral immune system (the spleen and the entire mucosa-associated lymphoid tissue, including the respiratory tract, the alimentary tract with Peyer's patches, separate follicles and caecal tonsils) exhibited simultaneously atrophic regressive changes in the spleen and varying degrees of lymphoid activity in the other areas. In two of the euthanised 14 days p.i. and three 21 days p.i. birds, a lymphoid proliferation of the nodular type within the mucosa-associated lymphoid tissue of the lungs was discovered.