Anthraquinones and anthraquinone derivatives were characterized for their antiviral and virucidal activities against viruses representing several taxonomic groups. One of these compounds, hypericin, had activity against vesicular stomatitis virus, herpes simplex virus types 1 and 2, parainfluenza virus, and vaccinia virus (from 0.5 to 3.8 log10 reductions in infectivity) at concentrations of less than 1 microgram/ml as determined by a direct pre-infection incubation assay. Human rhinovirus was not sensitive to hypericin at concentrations up to 10 micrograms/ml. Addition of small amounts of Tween-80 to solutions containing hypericin enhanced, by up to 2.6 log10, hypericin's virucidal activity. Anthraquinones and anthraquinone derivatives with the hydroxyl and alkyl substitution pattern of emodin (i.e. emodin, emodin anthrone, emodin bianthrone and hypericin) were active against the enveloped viruses tested. The following general pattern of activity was found: hypericin greater than emodin bianthrone greater than emodin anthrone greater than emodin. Chrysophanic acid, aloe-emodin, and sennosides A and B did not possess activity against any of the viruses tested.