Changes induced in primary dog kidney cell cultures by reovirus infection were investigated sequentially by virus and antigen estimations, conventional staining and direct fluorescent antibody techniques. Specific fluorescence was first detected 8-10 hours after infection and extended finally all over the cytoplasm. Fluorescence and cytoplasmic inclusion bodies were found to appear in five sequential stages distinguished by shape and location in the cell. These stages were in parallel to virus infectivity and hemagglutinating and complement-fixing antigenicity. Cytoplasmic immunofluorescence was also detectable in the throat smear of pups experimentally exposed to reovirus aerosols. The fluorescent antibody technique seemed to be a useful means for the rapid diagnosis of reovirus infection when applied to field cases.