The radiommunotherapeutic potential of 125I-labeled monoclonal antibodies was investigated in 48 nude mice (BALB/c, nu/nu) inoculated s.c. with the HeLa Hep 2 human adenocarcinoma cell line. This isotope, 125I, which is not commonly used for therapeutic purposes caused significant decrease in tumour growth from day 10 to day 42, when coupled to monoclonal antibodies directed against placental alkaline phosphatase (H7) or cytokeratins (TS1). The average growth rate was approximately 50-60% of that observed in the untreated control group after 42 days. The specific radioactivity in each organ 42 days after injection of radiolabeled monoclonal antibodies, indicated that these target antigens retain significant amounts of radiolabeled antibody in the tumours for at least 6 weeks after injection. No weight loss was seen in the animals during this experiment. By use of autoradiographic techniques, the labeled monoclonal antibodies were visualized deep in tumours in characteristic patterns representative of viable tumour cells (H7) and necrotic areas (TS1). The therapeutic approach using 125I labeled antibodies is encouraging and may offer new dimensions in radioimmunotherapy.