Clinical trials of radioimmunotherapy (RIT) of lymphoma have pro-duced frequent tumor regressions and remissions, but it has been diffi-cult to determine to what extent these tumor responses have been due to antibody-specific targeted radiation, nontargeted radiation, and/or cy-totoxicity mediated by the carrier monoclonal antibody (MoAb). In this report, RIT was studied in athymic nude mice bearing s.c. Raji human Burkitt's lymphoma xenografts using two different pan-B-cell MoAbs, MB-1 (anti-CD37) and anti-Bl (anti-CD20), which differ in isotype (and thus the potential for interaction with host effector mechanisms) and isotype-matched control antibodies either in the unlabeled state or labeled with 13,I. When a single i.p. injection of 300 μCi 131I-labeled MB-1 (IgGl) was compared to treatment with unlabeled MB-1 or 300 μCi 131I-labeled MYS control IgGl MoAb, an antibody-specific targeted radiation effect of RIT was seen. 131I-labeled MB-1 produced a 44 ± 19% (SEM) reduction in tumor size at 3 weeks posttreatment, while unlabeled MB-1 or 300 μCi 131I-labeled MYS control IgGl antibody treatment resulted in continued tumor growth over this period of time. In vitro studies demonstrated that MB-1 was incapable of mediating antibody-dependent cellular cytotoxicity using Raji tumor cell targets and human peripheral blood mononuclear cells. Similar to the MB-1 studies, treatment with 300 μCi 1111-labeled anti-Bl produced a 64% reduction in mean tumor size, while 300 μCi of control antibody resulted in a 58% increase in tumor size over the same 3-week period. In contrast to MB-1, however, unlabeled anti-Bl (an IgG2a MoAb which in vitro studies showed to be capable of antibody-dependent cellular cytotoxicity) also had a substantial antitumor effect. Indeed, 300 μCi 131I-labeled anti-Bl and unlabeled anti-Bl treatment (using an equivalent amount of total protein in the treatment dose) produced a similar specific reduction in tumor size. Increasing the radionuclide dose of anti-Bl to 450 μCi in another experiment did not produce a significant difference in tumor regression compared to a 300-μCi dose. These results suggest that the antitumor effects of 131I-labeled anti-Bl treatment were dominated by antibody-mediated cytotoxicity mechanisms, such that an antibody-specific targeted radiation effect could not be distinguished. In contrast, antibody-specific targeting of radiation was the dominant mechanism of tumor killing with 131I-la-beled MB-1. These results underscore the importance of investigating non-radiation-related antibody effects as well as radiation effects in ongoing lymphoma RIT trials with pan-B-cell antibodies. © 1992, American Association for Cancer Research. All rights reserved.