Viral inactivation is a critical step during the manufacture of factor VIII formulations for use in humans. Indeed, viral inactivation procedures may alter the three-dimensional structure of factor VIII resulting in the formation of new epitopes which, in turn, may lead to the synthesis of inhibiting antibodies, and hence to a decreased therapeutic activity. A rabbit model was used to compare the immunogenicity of a solvent/detergent (SD)-inactivated formulation with a double-inactivated (SD plus heating in solution, SDP) formulation of factor VIII. Two groups of five rabbits were immunized by six s.c. injections of each formulation. The serum of each animal was incubated with various amounts of the competing antigens, e.g. factor VIII SD or factor VIII SDP. The remaining free polyclonal antibodies were assayed by ELISA. Curves obtained with both antigens were compared for each serum. Both factor VIII SD and factor VIII SDP decreased the amount of antibodies raised to either formulation in a dose-dependent manner without observable differences. These results suggest that no new epitopes were present on factor VIII SDP as compared with factor VIII SDP as compared with factor VIII SD and that no epitope deletions occurred, supporting the view that the double-inactivation procedure does not change the immunogenicity of factor VIII SD. This model is proposed as a tool to detect changes in the immunogenicity of proteins which may be modified.