An ideal animal model for human immunodeficiency virus (HIV) infection in man should be caused by a genetically similar virus, it should cause as nearly identical clinical signs as possible, the pathogenesis of the resulting disease should be similar, and the model should be easily accessible to all researchers. Feline immunodeficiency virus (FIV) meets all these criteria. FIV is particularly useful as an animal model for investigation of the pathogenesis of lentivirus infections like cofactor studies, as well as for vaccine strategies, and drug testing in vitro and in vivo.