Summary: Lyophilization of formalin-inactivated Rift Valley fever virus vaccine, prepared in African green monkey kidney cell cultures, was successfully accomplished by the addition of 2% U.S.P. human serum albumin (HSA). The choice of HSA as an additive was made because it is non-allergenic for man and is not known to contain infectious hepatitis virus. Laboratory data have shown that the lyophilized vaccine containing 2% HSA is highly resistant to thermal deterioration and extremely stable in its immunogenic potency. These characteristics make stockpiling and shipment of the vaccine unrefrigerated entirely feasible, and permit the use of this product as a reference vaccine.