The progression of Gallibacterium anatis infection in immunosuppressed versus immunocompetent chickens was investigated. Before experimental infection, birds were treated with corticosterone for general immunosuppression, or 5-fluorouracil, cyclosporine-A, cyclophosphamide for depletion of specific leukocyte populations. Necropsy and sampling were performed at 0, 3, 7, 10 and 28 days post infection. The used drugs did not cause selected depletion of B cells, T cells, heterophils and monocytes/macrophages, as determined by quantification of leukocytes in blood and lymphoid organs using different technologies. Bacterial re-isolation and counts of colony forming units (CFU) showed that G. anatis colonization pattern in various organs, and the numbers of bacteria in trachea were not affected by immunosuppression. However, the treatments acutely increased CFU counts derived from the spleen, which demonstrates that chemically induced immunosuppression intensifies systemic multiplication of G. anatis in chickens.