The immunizing potency of an inactivated Rift Valley fever (RVF) vaccine prepared from RVF virus infected mouse brain and RVF infected cell culture was studied in cattle and sheep. Different doses and adjuvants were compared. In laboratory trials cattle and sheep developed neutralizing antibodies against virulent RVF virus; in cattle, antibodies were still detectable 9 mo. after immunization. Although the immunity produced was inadequate to prevent viremia after challenge, evidence of protection against clinical RVF was obtained. Field trials in sheep showed that vaccination induced good immunity in pregnant ewes.