T101 monoclonal antibody recognizes a pan-T-cell antigen present on normal T cells and also found in high concentrations in cutaneous T-cell lymphoma. We used this antibody, radiolabeled with 111In, in gamma-camera imaging to detect sites of metastatic cutaneous T-cell lymphoma in 11 patients with advanced disease. In all patients, [111In]T101 concentrated in pathologically or clinically detected nodes, including those in several previously unsuspected nodal regions. Concentrations (per gram of tissue) ranged from 0.01 to 0.03 percent of the injected dose and were consistently 10 to 100 times higher than previously reported on radioimmunodetection. Focal uptake was seen in skin tumors and heavily infiltrated erythroderma but not in skin plaques. The specificity of tumor targeting was documented by control studies with [111In]chloride or [111In]9.2.27 (anti-melanoma) monoclonal antibody. Increasing the T101 dose (1 to 50 mg) altered distribution in nontumor tissues. These studies suggest that imaging with [111In]T101 may be of value in identifying sites of cutaneous T-cell lymphoma. In contrast to the targeting of solid tumors, the mechanism of localization appears to be related to binding to T cells, which can then carry the radioactivity to involved sites.