The murine monoclonal antibody (mAb) 145-9 recognizes an epitope present on CA125 but different from the epitope defined by the mAb OC125. To evaluate the clinical usefulness of the 145-9 antibody, immunoscintigraphy was performed in ovarian cancer patients and the effect of circulating CA125 on tumor imaging was investigated. Two milligrams (74 MBq) of 111In-labeled 145-9 was injected intravenously into 11 patients with ovarian cancer. Pre-injection serum CA125 concentrations were between 166 U/ml and 7414 U/ml. Tumors were visualized in 10 of 11 patients. In two patients, lymph nodes that were not detected by other imaging modalities but were clinically suspected as metastases were visualized. There was no correlation between serum CA125 level and antibody uptake in the tumors. Immune complexes between the antibody and circulating antigen were observed in sera of all the patients, but the fraction of radioactivity in complex form did not correlate well with serum CA125 levels. The immune complexes survived in the circulation and the circulating radiolabel, including immune complexes, was still bound to solid-phase CA125. The plasma clearance rate and hepatic uptake of the antibody were not significantly affected by circulating CA125. In conclusion, the antibody 145-9 formed complexes with CA125 in vivo but this did not Compromise the outcome of antibody imaging. The antibody 145-9 can be used in immunoscintigraphy of ovarian cancer irrespective of serum CA125 level.