Summary: A comparison is made of the antigenicity of eastern equine encephalomyelitis (EEE) and rabies (CVS) vaccines prepared with β-propiolactone (BPL) and ultraviolet (UV) irradiation used singly and in combination. The resistance indices obtained with the inactivated vaccines prepared with BPL and UV combined are not diminished as compared with the vaccines containing active virus. The advantages demonstrated by the use of the combination are: a) a marked decrease in the amount of BPL required for complete virus inactivation; b) elimination of the “tailing effect” observed when either agent is used alone; and c) a marked increase in the margin of safety between the concentration required for complete inactivation and the maximum concentration which does not critically alter the antigenicity of the vaccine.