Hypericin, a potent antiviral agent, displays, in the absence of light and electron donors, an EPR signal which is attributed to a semiquinone-like radical, formed by intermolecular electron transfer. On irradiation with visible light the amplitude of the EPR signal increases significantly. This increase is highest (ca. 20 fold) in aqueous dispersions of the lysine salt of hypericin. Irradiation of hypericin water aggregates in the presence of oxygen generates superoxide radicals which may be registered by the spin trap technique. This finding implies that the free radicals of hypericin and superoxide radicals formed on photoirradiation of hypericin may play a hitherto unrecognized role in the biological activities elicited by hypericin both in vivo and in vitro.