The immunising 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 both cattle and sheep developed neutralising antibodies against virulent RVF virus and in cattle antibodies were still detectable 9 months after immunisation. Although the immunity produced was inadequate to prevent viraemia after challenge, evidence of protection against clinical RVF was obtained. Field trials in sheep showed that vaccination induced a good immunity in pregnant ewes.