Imaging of tumors by using radiolabeled monoclonal antibodies (MoAs) is hindered by the presence of background activity. To reduce this problem, the authors investigated the process of removing labeled MoAs from plasma at selected times by means of extracorporeal immunoadsorption. In seven patients with either lung or breast carcinoma, an indium-111-labeled murine antibody was intravenously administered. Six to 24 hours later, immunoadsorption was performed by passing the patients' plasma through a goat anti-mouse antibody column connected to a plasma separator. Whole-body computer images were obtained before and after the treatment. Blood pool activity in the images was reduced by an average of 59%, while tumor activity dropped by only 10%. Tumor-to-blood activity ratios therefore more than doubled, improving by an average of 121% between the pre- and posttreatment image sets. Eight of 12 areas of known disease and three areas of unknown but later documented disease were detected after the immunoadsorption process, while the three areas of unknown disease and three of the areas of known disease were not detected in the preclearance images. Thus, the feasibility of using extracorporeal immunoadsorption to improve MoA imaging of tumors was demonstrated.