A major obstacle to in vivo delivery of lentivirus or other retroviral vectors is their lability in the presence of serum. In vivo, these viral particles are rapidly destroyed by nonspecific complement-mediated degradation mechanisms. The eventual effective use of retroviral vectors for in vivo gene delivery would be greatly facilitated by the development of methods to protect the viral particles from such degradation. This protocol describes methods for the production of complement-stabilized lentiviral vectors either by pseudotyping the viral particles with a fusion envelope protein containing the complement-regulatory protein CD55 (decay accelerating factor, DAF) or by coassembly with the native DAF protein. An in vitro serum inactivation assay is also described.