Comparative histological and immunocytochemical studies were conducted on formalin-fixed tissues from chickens infected with avian influenza viruses of varying virulence. Results showed a distinct pattern of disease that depended on the virulence of the virus and the susceptibility of the birds. At 3 days post-intranasal inoculation with a highly virulent H7N7 virus, all 6-to-8-week-old specific-pathogen-free (SPF) birds were affected, and all developed pancreatic necrosis and encephalitis associated with specific immunoperoxidase staining. Other same-aged SPF birds were only occasionally affected 6 to 8 days after intravenous inoculation with almost avirulent H4N4, H6N2, or H3N8 virus. Specific lesions and immunoperoxidase staining were noted in the kidneys only. The H7N7 virus in older commercial birds and an H7N3 virus in young SPF and older commercial birds caused intermediate mortality rates at 4 to 11 days postinoculation, and there was a broad range of lesions and specific immunoperoxidase staining in the pancreas, brain, kidney, heart, and skeletal muscle. Two exceptional birds had immunostaining of small blood vessels throughout their bodies with or without lesions or staining in the tissues, which may have represented a transitory pre-localizing phase occurring in many birds. There was necrosis without virus antigen detection in the bursae, thymuses, and cecal tonsils, possibly secondary to stress or only transitory infection of virus. These data indicate that rapid, retrospective diagnosis of avian influenza in fixed tissues is possible by using an immunoperoxidase test on pancreas, brain, and kidney.