An E. coli-expressed fusion protein was used to study the immunogenicity of avian reovirus sigma3 protein in chickens. The protein induced the production of specific antibodies detectable by immunofluorescence, ELISA and immunoblot. The antibody titre was low as determined by ELISA and was negative by virus neutralization test. However, when tested in passive immunization studies, the chicken antiserum specific to this protein and control antiserum from chickens vaccinated with avian reovirus SI 133 strain showed some protection against virus-induced mortality in 1-day-old chicks. The results thus confirm the importance of humoral immunity against avian reovirus infection and indicate that sigma3 protein may play a role in the induction of protective antibodies.