We developed a photodynamic method to inactivate viruses in human fresh plasma. Single plasma bags were illuminated with visible light in the presence of low doses of phenothiazine dyes like methylene blue or toluidine blue. By this treatment the infectivity of different enveloped viruses including the causative agent of AIDS, HIV-1, was completely removable from the plasma. Non enveloped viruses, however, proved to be more stable. The activities of clotting factors and other plasma proteins were only slightly decreased. There was no indication that the procedure led to important structural modifications of plasma proteins. The dyes are photodynamically active at concentrations much lower than those at which they are therapeutically used as antidots in the treatment of methemoglobinemia.