Twenty-eight patients with assessable residual ovarian cancer after cytoreductive surgery and chemotherapy received intraperitoneal I-131 labelled monoclonal antibodies. There was no response in eight patients with tumour nodules greater than 2 cm, a partial response in two of the 15 patients with tumour nodules less than 2 cm, and a complete response in three of the other five patients with positive peritoneal washings. A further six patients received Y-90 labelled monoclonal antibodies for residual ovarian cancer. There was no response in one patient with nodules greater than 2 cm, and a partial response in one of the other five patients with tumour nodules less than 2 cm. The non-specific radiation dose in the peritoneal cavity from the infused isotope was measured by lithium fluoride thermoluminescent dosimetry (TLD). The radiation dose received by the peritoneal serosa was less than 500 cGy and was not sufficient to account for the observed tumour response. Significant bone marrow suppression was observed with I-131 activities greater than 120 mCi and with Y-90 activities greater than 13 mCi. The haemopoietic bone marrow is the dose-limiting organ in patients receiving radioimmunotherapy.