The purpose of this study was to develop a chicken model for studying the pathogenesis of avian polyomaviral infection. Ninety-two 1-day-old specific-pathogen-free (SPF) chicks were divided into the following three treatment groups: nontreated controls, treated with cyclosporine and treated with cyclophosphamide (CY). All birds were inoculated with polyomavirus; half were orally inoculated and half were inoculated by intraperitoneal (IP) injection. Spontaneous mortality and lesion development consistent with polyomaviral infection in psittacine birds occurred only in the CY-treated, IP-inoculated chicks. Immunohistochemistry confirmed that the intranuclear inclusions found microscopically were associated with polyomaviral antigen. Serology demonstrated postinoculation titers against polyomavirus in several experimental groups. One-day-old SPF chickens appear to be convenient models for investigating the pathogenesis of avian polyomavirus infection.