Human immunodeficiency virus (HIV), isolated from cultures of infected peripheral blood lymphocytes, was added to solutions of highly purified antithrombin III (AT-III), which was stabilized with 1 M citrate and 17 percent sucrose. The efficiency of heat inactivation of HIV in AT-III at 60 degrees C was compared with that of HIV in culture medium and followed for periods from 0 to at least 10 hours. The virus added, titer 10(5) by ID50, was inactivated as rapidly (less than 30 minutes) and efficiently (completely) in the stabilized AT-III as in the culture medium. Virus was determined both by direct measurement of the HIV-related reverse transcriptase activity and by quantitation of virus infectivity by ID50 assay.