Recent studies in animal models involving antibody tumor targeting of hepatoma and melanoma and clinical trials involving hepatoma patients have suggested that preirradiation of tumors may enhance antibody tumor targeting. These reports led us to study the effect of external irradiation on monoclonal antibody (MAb) targeting of human carcinomas; as a model system, we used MAb B72.3 and the LS-174T human colon carcinoma xenograft in athymic mice. LS-174T tumors exposed to 300 cGy grew to approximately 93% the size of non-irradiated tumors, while those exposed to 600, 900, or 2,000 cGy were approximately 41% the size of control tumors. Splitting the 900 cGy into three 300-cGy fractions yielded a two-fold lower tumor volume compared with a single 900-cGy fraction. Histochemical evaluation of the carcinomas revealed a decrease in the number of mitoses per high power field consistent with early effects of radiation exposure. Using the avidin-biotin complex immunoperoxidase technique, carcinomas were assayed for expression of the tumor associated glycoprotein (TAG)-72, the high-molecular-weight mucin detected by MAb B72.3. No discernable variation was observed in the staining intensity among tumors in both the control and radiation treated group; that is, differences among tumors within each group were compatible with the known heterogeneous expression of TAG-72. Exposure of carcinomas to 300 or 900 cGy in a single fraction or 900 cGy split in three 300-cGy fractions did not yield a consistent or substantial enhanced localization of radiolabeled MAb B72.3 IgG or F(ab')2 to tumors. A 1.5-fold augmentation of MAb binding to tumors was observed in preirradiated mice; however, these results were not statistically significant. Inherent differences in tumors such as cell type of origin, size, spatial configuration, extent of vascularization and volume of interstitial space may contribute to variability of the effect of preirradiation of tumors on antibody binding. Our results suggest that consistent augmentation of radiolabeled antibody localization to tumors is not a universal phenomenon.