A combined method has been developed for selective cytolysis in vitro as well as in vivo using a photosensitizer haematoporphyrin-protein conjugate as the targeting molecule and low-power He-Ne laser (632.8 nm) irradiation in order to activate the sensitizer to its excited, toxic triplet energy state. The specificity of the procedure was demonstrated in vitro by purging a mixed cell population from one component, and in vivo in an animal (nude mice) xenograft tumour model, where human cancer cells were destroyed by the immunotargeting method using monoclonal-antibody-haematoporphyrin (mAb-HP) conjugate (a-PNAr-I mAbs, which bind to the cell surface antigens of gastric cancer cells) and soft laser irradiation. The cell destruction was dependent on the doses of both mAb-HP and He-Ne laser light energy, and occurred only in target cell populations.