A radiolabeled murine monoclonal antibody (T101) was used for imaging and therapy of six patients with cutaneous T cell lymphoma (CTCL). Radioimmunodetection was performed with a 5.6 to 13.1 mCi 131I-T101 preparation (9.6 to 10.5 mg). A therapeutic dose of 100.5 to 150.1 mCi 131I on 9.9 to 16.9 mg of antibody was administered to five patients, with subsequent retreatment following plasmapheresis in three patients at the time of disease progression. All patients responded to their initial therapy and two patients responded to retreatment. Regression of skin lesions and peripheral adenopathy was witnessed. All patients reported resolution of their chronic pruritus. The duration of response ranged from 3 weeks to 3 months. Acute toxicity included fevers, pruritus, and mild dyspnea in two instances. Myelosuppression was seen in patients receiving the 144.7 mCi, 145.0 mCi, and 150.1 mCi 131I-T101 doses. Radioimmunodiagnostic and therapy studies included gamma scintigraphy, plasma, urinary, and wholebody antibody clearances, and biodistribution determined from skin, bone marrow, and liver biopsies. Immunologic studies included immunoperoxidase staining of target tissues, immunofluorescent flow cytometric analysis on peripheral blood and bone marrow, assays for serum blocking factors, determination of a human antimouse antibody (HAMA) response, and quantitation of circulating T101 levels. These preliminary data suggest that 131I-T101 has therapeutic potential in CTCL and that myelosuppression will be the limiting toxicity.