The virus that causes the acquired immunodeficiency syndrome (AIDS), human T lymphotropic virus/lymphadenopathy-associated virus (HTLV-III/LAV), was incubated at temperatures from 37 degrees to 60 degrees C and virus titer (ID-50) was determined over time by a microculture infectivity assay. The rate of thermal decay was consistent with first-order kinetics, and these data were used to construct a linear Arrhenius plot (r = 0.99), which was used to determine inactivation time as a function of temperature. In the liquid state, thermal decay was little affected by matrix (culture media, serum, or liquid Factor VIII). In the lyophilized state, the time required to reduce virus titer 10-fold (1 log) at 60 degrees C was 32 min compared with 24 s in the liquid state. HTLV-III/LAV in liquid antihemophilic Factor VIII or IX was lyophilized and heated according to commercial manufacturers' specifications. Infectious virus was undetectable with these regimens. Heat treatment should reduce or stop transmission of HTLV-III/LAV by commercial antihemophilic Factor VIII or IX.