Virus resistance to antiviral therapies is an increasing concern that makes the development of broad-spectrum antiviral drugs urgent. Targeting of the viral envelope, a component shared by a large number of viruses, emerges as a promising strategy to overcome this problem. Natural and synthetic porphyrins are good candidates for antiviral development due to their relative hydrophobicity and pro-oxidant character. In the present work, we characterized the antiviral activities of protoprophyrin IX (PPIX), Znprotoporphyrin IX (ZnPPIX), and mesoporphyrin IX (MPIX) against vesicular stomatitis virus (VSV) and evaluated the mechanisms involved in this activity. Treatment of VSV with PPIX, ZnPPIX, and MPIX promoted dose-dependent virus inactivation, which was potentiated by porphyrin photoactivation. All three porphyrins inserted into lipid vesicles and disturbed the viral membrane organization. In addition, the porphyrins also affected viral proteins, inducing VSV glycoprotein cross-linking, which was enhanced by porphyrin photoactivation. Virus incubation with sodium azide and ␣-tocopherol partially protected VSV from inactivation by porphyrins, suggesting that singlet oxygen ( 1 O 2 ) was the main reactive oxygen species produced by photoactivation of these molecules. Furthermore, 1 O 2 was detected by 9,10-dimethylanthracene oxidation in photoactivated porphyrin samples, reinforcing this hypothesis. These results reveal the potential therapeutic application of PPIX, ZnPPIX, and MPIX as good models for broad antiviral drug design.KEYWORDS photoactivation, porphyrin, singlet oxygen, vesicular stomatitis virus, viral inactivation D espite the increasing number of antiviral drugs available nowadays, therapeutics for viral diseases often fail due to drug resistance development by the target viruses. In the search for antiviral agents with a broad spectrum of activity against viruses, antiviral agents targeting the viral envelope, a component shared by a large number of viruses, emerge as promising compounds able to overcome the drug resistance problem. Porphyrins, amphipathic molecules able to interact with membranes, appear to be good candidates for the development of such antivirals. Porphyrins are formed by four pyrrole rings linked by methine bonds. The nitrogen atoms in the center of the ring form a pocket able to coordinate metallic ions, conferring to porphyrin the ability to absorb light at additional wavelengths and act in catalytic transformations (1). These activities allow porphyrins to be involved in important biological processes, such as respiration (heme) and photosynthesis (chlorophyll and bacteriochlorophyll) (2).