Lyophilized plasma derivatives are more stable to heat than when they are in the liquid state. Commercial Factor VIII (antihemophilic factor) was seeded with a measured quantity of hepatitis B virus. The contaminated material was then lyophilized and subjected to heat of 60°c for 30 hr. Chimpanzees were inoculated with the heat-treated antihemophilic factor or sham-treated antihemophilic factor that had been held at 4°c. Surprisingly, hepatitis B virus survived the heating procedure with no apparent loss in titer: the incubation period to appearance of HBsAg was that expected for the challenge dose of virus. Even more surprising, one chimpanzee (the recipient of the unheated antihemophilic factor) also developed non-A, non-B hepatitis and two chimpanzees (recipients of the heated antihemophilic factor) also developed delta hepatitis. Neither of these agents was a contaminant of the hepatitis B virus challenge pool, since the purity of this hepatitis B virus pool was established previously in chimpanzees. Thus, both a non-A, non-B agent and the delta agent apparently contaminated the commercial antihemophilic factor. This is the first direct evidence for contamination of antihemophilic factor with the delta agent and confirms previous seroepidemiologic evidence for its presence in pooled plasma derivatives. Subsequent inactivation studies were performed with antihemophilic factor experimentally contaminated with the Hutchinson strain of non-A, non-B hepatitis virus. In these studies, heating at 60°c for 30 hr in the dry state rendered antihemophilic factor free of detectable non-A, non-B hepatitis virus.