In vivo tumor targeting and biodistribution of radiolabeled anti-CEA Fab-H-2K^b/ova conjugates. Demonstration of the specific tumor uptake of radiolabeled anti-CEA-H-2K^b/ova conjugate, as compared with control F(ab')₂ fragments, injected i.v. into (A) three CEA transgenic C57BL/6 mice bearing a subcutaneous graft of the syngeneic colon carcinoma MC38-CEA+, and (B) four nude mice each bearing on one lateral side a subcutaneous graft of the same MC38-CEA+ cells and on the other side a second subcutaneous graft of the CEA-expressing human LS174T colon carcinoma cells. All mice were coinjected with 2 µg ¹²⁵I-labeled anti-CEA Fab-H-2K^b/ova conjugate and with a control F(ab')₂ fragment labeled with ¹³¹I. For the CEA transfected mice the control F(ab')₂ had an irrelevant specificity and for the nude mice the control F(ab')₂ was derived from an anti-human EGF receptor mAb. Mice were sacrificed 24 h after injection and the radioactivity for both iodine isotopes was measured in tumors and normal tissues. Results expressed in percent of injected dose per g tissue (% ID/g) for anti-CEA Fab-H-2K^b/ova conjugate (black bars) and for the control F(ab')₂ fragments (open bars) show that the anti-CEA conjugate specifically targeted the CEA-expressing tumors while the irrelevant F(ab')₂ did not, and the anti-human EGF receptor targeted only the human colon carcinoma xenograft but not the MC38-CEA+ murine carcinoma.