Photodynamic therapy of tumors such as bladder tumors includes photosensitizing with a photosensitizer that is a complex or a compound of hypericin and a polymeric complexing agent. The photosensitizer is formed from an alkali salt of hypericin and the polymeric complexing agent. The alkali salt of hypericin is a sodium salt or a potassium salt. The complexing agent is a polyethylene glycol or a poly-N-vinyl amide.