BACKGROUND AND OBJECTIVES: Alterations of factor VIII (FVIII) during preparation procedures can potentially affect its immunogenicity. One method evaluating such alterations could be by determining the reactivity of FVIII with specific antibodies. MATERIALS AND METHODS: Since heat treatment is currently used to reduce the risk of viral transmission, we evaluated the immunoreactivity of plasma-derived FVIII before and after heating at different temperatures and for different periods. Freeze-dried FVIII was used for these experiments as part of the validation procedure of a novel FVIII preparation. RESULTS: Heating FVIII for up to 72 h at 80 degrees C does not alter its reactivity with specific rabbit antibodies or mouse monoclonal antibodies, although some loss of FVIII activity occurred after 72 h. After heating for 2 h at 100 degrees C, a procedure that reduced FVIII activity by about 50%, there were still no significant effects on FVIII reactivity with monoclonal antibodies. CONCLUSIONS: Freeze-dried preparations of plasma-derived FVIII seem to be resistant to heat-induced structural denaturation.