The anti-human epidermal growth factor receptor (EGF-R) humanized monoclonal antibody (MAb) h-R3 is an (IgG1), which binds to an extracellular domain of EGF-R. It was used to evaluate the biodistribution on nude mice xenografted with H-125 human lung adenocarcinoma cell line. Results were compared with its murine version of the MAb ior-egf/r3. Twenty-one athymic female 4NMRI nu/nu mice were injected intraperitoneally with 10 microg/100 muCi of 99mTc-labeled MAbs. Immunoreactivity of 99mTc-labeled MAbs were measured by enzyme-linked immunosorbent assay (ELISA) on H-125 cell line and the immunoreactive fractions was determined by the Lindmo method. Among all organs, significant accumulation was found in serum (27.05 +/- 2.08 %ID/g) and tumor (3.903 +/- 0.89 %ID/g) at 4 h after injection. These values decreased to 5.03 +/- 0.50 %ID/g and 2.19 +/- 0.56 %ID/g for serum and tumor, respectively. The immunoreactive fraction was found to be 0.70, with a correlation coefficient r = 0.9984. With the good biodistribution and tumor uptake of the 99mTc-labeled humanized antibody h-R3, a phase I diagnostic clinical trial of tumor with epithelial origin should be pursued.