The purpose of this study was to determine if proton irradiation can increase the localization of radiolabeled monoclonal antibodies (MAb) in subcutaneous (s.c.) or intracranial (i.c.) human lung tumors xenotransplanted in athymic rats. Rats with carcinoembryonic antigen (CEA)-expressing (NCI-H441) tumors were irradiated using 3 different proton time-dose regimens, followed by 111In-ZCE025, an anti-CEA MAb, which was injected 2 hr after the last dose of irradiation, and the animals were euthanized 3 days later for biodistribution and other assays. Proton irradiation at 10 gray (Gy) as a single dose or in 2 Gy fractions given on 5 consecutive days increased the uptake of 111In-ZCE025 into s.c. tumors by 292% and 182%, respectively, compared to nonirradiated controls. No enhancement in radiolabeled MAb delivery was seen after hemibrain irradiation in animals with i.c. tumors. Histopathological examination of both implantation sites showed a viable poorly differentiated adenocarcinoma with a decrease in blood vessel density, a decrease in mitotic activity, and an increase in areas of necrosis following irradiation as compared with adjacent nonirradiated tissue. CEA expression was generally maintained in vivo in that the marker was detectable in the tumor, plasma, and cerebrospinal fluid. Oxygen radical production by peripheral blood cells from s.c. and i.c. tumor-bearing rats exhibited strikingly different patterns of responsiveness. I.c. injected animals were 24% lighter than their s.c. injected counterparts, but no neurological signs of tumor progression were noted. The results indicate that proton irradiation can be used effectively to increase the delivery of radiolabeled MAb to s.c. implanted human lung tumor xenografts. However, in order to accomplish this in the brain, other radiation time-dose schedules and treatments may be needed.