Avian reovirus (ARV) and avian nephritis virus (ANV) were individually isolated from runty 10-day-old broiler chicks. The ARV isolate, IR-R, the ANV isolate, IR-N, and the reference strain of ANV, G-4260, were inoculated orally into 1-day-old chicks of two specific-pathogen-free (SPF) chicken lines, 151 and PDL-1. Growth retardation without the presence of gross lesions was clearly observed at 7 and 14 days postinoculation (PI) in chicks of both lines inoculated with the IR-R strain. On the other hand, in chicks inoculated with IR-N strain, growth retardation was observed only in the chicks of line 151 at 7 and 14 days PI. Microscopically, nephritis was observed in both chicken lines at 7 and 14 days PI. When chicks that were inoculated with the IR-N strain at 1 day of age were inoculated with the IR-R strain at 3 days of age, growth retardation was observed in the chicks of line PDL-1 at 10 and 17 days PI. However, the growth retardation was less severe than in the group receiving a single inoculation of the IR-R strain.