Ibaraki virus grown in HmLu-1 cells, a stable cell line from hamster lung, agglutinated erythrocytes from cattle, sheep, goats, horses and rabbits, when concentrated by ultrafiltration from infectious culture fluid. The hemagglutination (HA) was dependent on not only the pH but also the NaCl concentration of the diluent; the HA titer significantly improved by increasing the NaCl molarity to 0.6 M and standardizing pH to 7.5. The incubation temperature did not affect the HA titer. In Sepharose 4B gel filtration the peak of the HA activity coincided in position with the infectivity peak. The HA reaction was inhibited by specific antiserum. Some factors involved in the HA and HA-inhibition (HI) were investigated and standard HA and HI tests were established. HI antibody titers of individual bovine sera showed a significant positive correlation with their neutralizing antibody titers.