To improve the detectability of tumors by light-induced fluorescence, the use of monoclonal antibodies (MoAb) as carriers of fluorescent molecules was studied. As a model for this approach, the biodistribution of an anticarcinoembryonic antigen (CEA) MoAb coupled to fluorescein was studied in mice bearing a human colon carcinoma xenograft. In vitro, such conjugates with fluorescein-MoAb molar ratios ranging from four to 19, doubly labeled with lZ5I, showed more than 82% binding to immobilized CEA. In vivo, conjugates with a fluorescein-MoAb molar ratio of ten or less resulted in a tumor uptake of more than 30% of the injected dose of radioactivity per gram tumor at 24 hours. Tumor to liver, kidney, and muscle ratios of 20, 30 and 72, respectively, were obtained 48 hours after injection of the lz5I-MoAb-(fluorescein),, conjugate. The highest fluorescence intensity was always obtained for the tumor with the anti-CEA MoAb conjugate; whereas in control mice injected with fluoresceinated control immunoglobulin G,, no detectable increase in tumor fluorescence was observed. To compare these results with a classically used dye, mice bearing the same xenografts received 60 pg of Photofrin 11. The intensity of the fluorescence signal of the tumor with this amount of Photofrin I1 was eight times lower than that obtained after an injection of 442 ng of fluorescein coupled with 20 pg of MoAb, which gave an absolute amount of fluorescein localized in the tumor of up to 125 ng/g of tumor. 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 MoAb directed against tumor markers. Cancer 67:2529-2537,1991. ETECTION OF SMALL TUMORS is difficult and can D play an important role in the overall prognosis of several cancers. The use of dyes has been shown to be From the