The transferrin receptor is often highly expressed in tumor cells whereas it is usually present at low levels in surrounding normal adult tissue. Here, a potential anti-cancer agent is described, which is directed at this receptor and consists of a toxin-modified transferrin, which is activated via photodynamic therapy. The porphyrin chlorin e6 was conjugated to transferrin using a procedure, which involved the preliminary binding of the protein to quaternary amino ethyl-sephadex. This maintained the natural activity of the transferrin, and the un-activated conjugate exhibited no in vitro cellular toxicity. The conjugate's singlet oxygen yield was estimated by assessment of its light-dependent oxidation of tetramethylbenzidine, where it displayed approximately 70% of the efficiency of native chlorin e6. When chlorin e6-transferrin treated human MCF7 and rat MTLn3 mammary adenocarcinoma cells were exposed to toxin-activating visible light, a tumor cell killing effect was achieved in normal (medium plus 10% FBS) culture conditions with an ED50 of approximately 10-20 microg/ml. A method for the synthesis of chlorin e6-transferrin was developed, and the conjugate was shown to exhibit a light-dependent killing of mammary adenocarcinoma cells in culture. The conjugate demonstrated potential as an anti-cancer agent.