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To clarify the usefulness of fluorescent diagnosis for gastric cancer, we assessed the sensitivity arid specificity of the tumor-localizing photochlorine photosensitizer ATX-S10 in combination with a new fluorescence diagnostic system. Into the submucosa of the stomachs of five rabbits, VX-2 tumor cells originating from squamous cell carcinoma were injected. After 3 weeks, three rabbits (Group I) were sacrificed 3 h after intravenous injection of ATX-S10 at a dose of 20 mg/kg, and their stomachs were observed by the Hg-lamp-induced fluorescence diagnostic system. The other two rabbits (Group II) were also sacrificed without injection of ATX-S10 and observed-by the same method. In all cases in Groups I and II, gastric cancers that were invaded from the submucosa to the serosa were recognized histologically. The concentration of ATX-S10 examined by high-performance liquid chromatography (HPLC) in the gastric cancer was significantly higher than in the normal stomach. Fluorescent spectroscopy could detect 630 nm fluorescence selectively, consistent with the cancerous tissue of Group I, Moreover, fluorescent images were detected in only the exposed area of cancerous tissue and were undetected in the surrounding normal mucosa, Conversely, no fluorescent images could be detected in the stomachs of Group II. It is suggested that a fluorescence diagnostic system using ATX-S10 may become useful for the diagnosis of the existence or extension of carcinoma of the gastrointestinal tract. (C) 1997 Elsevier Science Ireland Ltd.