The aim of the present study was to investigate the pathology in normal or immunosuppressed chickens followed intravenous or intraperitoneal inoculation with a well-characterized strain of Gallibacterium anatis. Two groups of 30 15-week-old commercial brown laying chickens were used, having been screened and found negative for Gallibacterium organisms. One group was treated with 5-fluorouracil to promote heterophil depletion, while the other was saline treated. Ten days later 15 chickens from each group were inoculated either intravenously or intraperitoneally with 3.3 x 10(7) colony-forming units of G. anatis strain 12656-12. Subsets of chickens were sacrificed at 3, 12 or 24 h post-infection and examined for lesions. Livers and spleens were examined by culture and by fluorescent in situ hybridization. Intravenously infected birds showed severe septicaemic lesions in both the normal and immunosuppressed birds. Mortality was recorded only in the latter, with an overall rate of 73%. The intraperitoneally infected chickens of normal immune status showed various degrees of localized purulent peritonitis at the inoculation site, but in the immunosuppressed birds the entire peritoneum tended to be involved along with the abdominal organs. This was similar to previous descriptions of natural infections and may represent a useful infection model for detailed analysis of Gallibacterium virulence factors and pathogenesis.