Intraoperative radioimmunodetection of ovarian cancer using monoclonal antibody B72.3 and a portable gamma-detecting probe. To assess the value of radioimmunoguided surgery in the intraoperative detection of ovarian cancer, we used monoclonal antibody B72.3, radiolabeled with 125I, and a hand-held gamma-detecting probe in 13 women with ovarian cancer undergoing exploratory laparotomy. B72.3, which recognizes a tumor-associated glycoprotein, TAG 72, was injected 12-29 days preoperatively (intraperitoneally in four cases, intravenously in nine, and by both routes in one). Intraoperatively, the abdomen was surveyed with the probe and probe counts were correlated with biopsies and excised specimens studied by routine histologic stains. Probe counts were positive in four of seven evaluable patients with histologically confirmed disease. In three of these four patients, the probe detected cancer in specimens interpreted as normal on frozen histologic sections. The probe also identified microscopic cancer in the one patient who had no gross disease. The specificity of the probe was 70%. Preoperative computed tomography was normal in all patients, including those with tumors as large as 3 cm. This pilot study shows the ability of radioimmunoguided surgery to detect occult ovarian cancer. 