The application of intraperitoneal (i.p.) radioimmunotherapy to treat i.p. tumor loci has been limited by bone marrow toxicity secondary to circulating radiolabeled antibodies. The generation of novel genetically engineered monoclonal antibodies, which can achieve high tumor uptake and rapid blood clearance, should enhance the therapeutic index of i.p. radioimmunotherapy. In this regard, a novel humanized anti-TAG-72 monoclonal antibody with a deleted CH2 region (HuCC49ΔCH2) has been described, which localized well to subcutaneous xenograft tumors and had a rapid plasma clearance. The aim of this study was to examine the characteristics of this radiolabeled reagent when administered through the i.p. route in mice bearing i.p. tumor (LS174T). The ΔCH2 molecule and intact humanized CC49 (HuCC49) monoclonal antibody were conjugated to PA-DOTA and radiolabeled with 177Lu. Both molecules retained high-affinity binding to TAG-72 positive LS174T tumor cells in vitro. The radiolabeled ΔCH2 molecule had a modest decrease in tumor localization, as compared to the intact molecule when administered i.p. to tumor-bearing mice and a dramatically shorter plasma disappearance T 1/2 at 2.7 hours compared to 61.2 hours for the intact antibody. The radiolabeled ΔCH2 molecule thus had very high tumor:blood ratios. Using an 131I-labeled system, the maximum tolerated dose of ΔCH2 was >3× that of intact HuCC49. Autoradiography of tumors showed low radiation dose rates at tumor centers early (1 and 4 hours), as compared to higher dose rates at tumor periphery but a more uniform distribution by 24 hours. Dose-rate distributions were similar for both reagents. Animals bearing LS174T i.p. tumors were treated with 300 μCi of 177Lu-labeled ΔCH2 or intact HuCC49 by i.p. route daily × 3. The 177Lu-ΔCH2 molecule mediated an increase in median survival compared to controls (67.5 ± 7.5 days versus controls of 32 ± 3.3) while the same dose of 177Lu-HuCC49 produced early toxic deaths. These studies suggest that i.p. radioimmunotherapy using radiolabeled HuCC49ΔCH2 should allow higher radiation doses to be administered with less marrow toxicity and potentially improved efficacy. © Mary Ann Liebert, Inc.