Lymphoid cells infiltrating into human tumors can be expanded in vitro in medium containing interleukin-2 (IL-2). Adoptive transfer of these tumor-infiltrating lymphocytes (TIL) mediates potent antitumor effects in murine tumor models. Clinical trials to evaluate the efficacy of these cells in patients with advanced cancer are underway. We have investigated whether infused TIL labeled with indium 111 (111In) oxine can traffic and localize to metastatic deposits of tumor. Six patients with metastatic malignant melanoma who had multiple sites of subcutaneous, nodal, and/or visceral disease were the subjects of the study. The patients received cyclophosphamide 36 hours before receiving the intravenous (IV) infusion of TIL followed by IL-2 IV every eight hours. The distribution and localization of the TIL were evaluated using serial whole body gamma camera imaging, serial blood and urine samplings, and serial biopsies of tumor and normal tissue. 111In-labeled TIL localized to lung, liver, and spleen within two hours after the infusion of activity. Activity in the lung diminished within 24 hours. As early as 24 hours after injection of 111In-labeled TIL, localization of TIL to sites of metastatic deposits was demonstrated in all six patients using either imaging studies or biopsy specimens or both. 111In activity in tumor tissue biopsies ranged from three to 40 times greater than activity in normal tissue. A progressive increase in the radioactive counts at sites of tumor deposit was seen. This study shows that labeled TIL can localize preferentially to tumor, and provides information concerning the possible mechanism of the therapeutic effects of TIL.