The use of dyes has been helpful for the photodiagnosis of small cancers accessible to endoscopic examn. An important limiting factor of this technol. is that the presently used fluorescent dye mixt. has a relatively poor capacity to accumulate preferentially in malignant tissue and a low quantum yield of fluorescence. To improve these parameters, fluorescein was coupled to an anti-carcinoembryonic antigen (CEA) MAb and the biodistribution of several conjugates was studied in nude mice bearing a human colon carcinoma xenograft. In vitro, such conjugates with fluorescein to MAb molar ratios ranging 4-19, trace labeled with 125I, showed >82% binding to insolubilized CEA. However, since the aim of this work was the evaluation of MAb-dye conjugates designed for in vivo tumor localization, all newly prepd. MAb-fluorescein conjugates were tested in the exptl. model of nude mice bearing CEA expressing human colon carcinoma xenografts. Under these exptl. in vivo conditions, the conjugate contg. 10 fluorescein mols. per MAb mol. gave an excellent tumor localization (up to 30% of the injected dose per g tumor at 24 h), whereas a conjugate with 19 fluorescein mols. per MAb mol. gave almost no in vivo localization in the tumor, probably due to a very short half-life. Tumor-to-liver, -kidney, and -muscle ratios of 20, 30, and 72, resp., were obtained at 48 h after injection of the conjugate contg. 10 fluorescein mols. per MAb mol. In the spectrofluorometric anal., a high fluorescence intensity was obsd. in the tumor after injection of the anti-CEA MAb conjugate. To compare these results with a conventionally used dye, mice bearing the same xenografts received a purified form of hematoporphyrin, Photofrin II. The intensity of the fluorescence signal of the tumor after an injection of 0.44 mg fluorescein coupled to 20 mg of MAb was 8-fold higher than that obtained after injection of 60 mg of Photofrin II. These results illustrate the possibility of improving the specificity of in vivo tumor localization of dyes for laser-induced fluorescence photodetection and phototherapy by coupling them to MAb directed against tumor markers.