Because of the cumbersome conditions experienced in a maximum containment laboratory methods for inactivating highly pathogenic viruses were investigated. The infectivity of Lassa, Marburg, and Ebola viruses was inactivated without altering the immunological activity after radiation with Co 60 gamma rays. At 4°C, Lassa virus was the most difficult to inactivate with a rate of 5.3 x 10(6) log 50% tissue culture infective dose per rad of Co 60 radiation. as compared with 6.8 x 10(6) log 50% tissue culture infective dose per rad for Ebola virus and 8.4 x 10(6) log 50% tissue culture infective dose per rad for Marburg vius. Experimental inactivation curves, as well as curves giving the total radiation needed to inactivate a given concentration of any of the three viruses, are presented. We found this method of inactivation to be superior to UV light or beta-propiolactone inactivation and now routinely use it for preparation of material for protein chemistry studies or for preparation of immunological reagents.