Hypericin (Figure 1) has been shown by Lavieet al. toinactivate mature and properly assembled retroviruses, notably human immunodeficiency virus (HIV).' Recently Kraus, Carpenter, and co-workers demonstrated the antiretroviral activity of hypericin against equine infectious anemia virus (EIAV), a lentivirus closely related to HIV, and have determined that light is required for activity.2 The requirement of light leads to the fundamental questions, what is the mechanism of action of hypericin and what is the role of light?