Six strains of equine infectious anemia (EIA) virus propagated in equine leukocyte cultures were found to agglutinate horse erythrocytes. Concentrated virus material containing about 20 units of complement fixation (CF) titer showed hemagglutinating (HA) titers ranging from 4 to 8 units. The HA activity remained stable after ether treatment and was reduced by trypsin, formaldehyde and KIO4. Cesium chloride equilibrium density gradient centrifugation revealed two populations of hemagglutinin, one in the density range of 1.15-1.16 g/ml coinciding with a peak of CF antigen and the other at round 1.27 g/ml. However, after the antigen was treated with ether, hemagglutinin showed a single peak at about 1.27 g/ml. Hemagglutinin receptors on the erythrocytes were inactivated by trypsin and formaldehyde but their activity was enhanced by neuraminidase. Hemagglutination was inhibited by sera from horses infected with homologous strain for EIA virus. The hemagglutinin showed immunological properties similar to those of the hemagglutinin of guinea pig erythrocytes as reported in our previous paper.