Twenty-four patients with persistent epithelial ovarian cancer after chemotherapy with or without external beam irradiation, were treated with intraperitoneally administered 131I-labeled monoclonal antibodies HMFG1, HMFG2, AUA1, H17E2, directed against tumor-associated antigens. Acute side effects were mild abdominal pain, pyrexia, diarrhea, and moderate reversible pancytopenia. One patient developed a subphrenic abscess requiring surgical drainage. Eight patients with large volume disease, ie, greater than 2 cm tumor diameter, did not respond to antibody-guided irradiation and died of progressive disease within 9 months of treatment. Sixteen patients had small-volume (less than 2 cm) disease at the time of treatment with radiolabeled antibody. Seven patients failed to respond, and of nine initial responders, four patients remain alive and free from disease 6 months to 3 years from treatment. Analysis of the data on relapse indicated that doses greater than 140 mCi were more effective than lower doses. We conclude that the intraperitoneal administration of 140 mCi or more of 131I-labeled tumor-associated monoclonal antibodies represents a new and potentially effective form of therapy for patients with small-volume stage III ovarian cancer.