Neutralizing (NT) and hemagglutination-inhibiting (HI) antibodies to calf diarrhea coronavirus (CDCV) and hemagglutinating encephalomyelitis virus of swine (HEV) (strain 67N) were detected in high proportions of normal adult cattle and pigs in Japan. Since comparison of NT and HI titers in the serum samples suggested in antigenic difference between the viruses, cross NT and HI tests of these viruses were carried out with antisera raised in rabbits. The homologous NT titers were markedly higher than the heterologous titers. In HI tests essentially the same results were obtained. These findings indicate the presence of a marked difference in antigenic make-up between CDCV and HEV. NT and HI tests can clearly differentiate the viruses, although there is some cross reaction.