Equine arteritis virus (EAV) grown on RK13 cell cultures was tested for hemagglutination (HA) with erythrocytes from a variety of species at 4 degrees C, room temperature and 37 degrees C. HA was observed at all temperatures with erythrocytes from mouse and chicken but not with those of cattle, horse, rabbit, guinea pig, mongolian gerbil, goose or chick embryo. Chickens showed an individual variation in agglutinability of their erythrocytes, requiring selection of birds to obtain erythrocytes for HA. The HA activity was enhanced by treatment of virus materials with Tween 80 followed by treatment with ether. The HA reaction was inhibited by specific antiserum. Higher HA-inhibiting (HI) antibody titers were obtained by the incubation of serum-HA antigen mixture at 4 degrees C for 24 hr. HI antibody titers of individual horse sera showed a significant positive correlation with their neutralizing antibody titers.