Bovine ephemeral fever virus of low passage level (5th to 8th) in tissue culture has proved to be more antigenic for cattle than higher passage level (17th to 19th) or mouse-brain-adapted virus. However, the neutralizing antibody response appears to be short-lived and therefore probably provides inadequate protection. Similar results were obtained when ephemeral fever virus of both low and high passage level was adsorbed onto aluminium hydroxide and inoculated into cattle. Emulsions of ephemeral fever virus of both low and high passage level in Freund's incomplete mineral oil adjuvant induced high neutralizing antibody titres in vaccinated cattle. These antibodies were detectable for at least a year and the animals were found to be immune when infected with virulent bovine ephemeral fever virus.