Abstract: Objective To study the specific killing effect of the Photofrin-Herceptin immunoconjugate on the tumor cells expressing HER-2. Methods Photofrin (porfimer sodium) was covalently coupled to Herceptin (trastuzumab) via [1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide hydrochloride](EDCI), and the killing effect of this conjugate was evaluated by means of MTT assay on SK-BR-3, MCF-7 and A549 cell lines expressing HER-2 at different levels. Results The conjugate had weaker immunoactivity than Herceptin, but possessed stronger killing effect on the cell lines expressing HER-2 than Photofrin, Herceptin, as well as the mixture of Photofrin and Herceptin (P<0.05). In the cell lines negative for HER-2 expression, the killing effect of the conjugate was similar to that of Photofrin, Herceptin, and their mixture. Conclusion Photofrin-Herceptin immunoconjugate can specifically kill HER-positive cells in vitro.