Basic study was performed to evaluate the efficacy of immunoscintigraphy of ovarian cancer using 99mTc labeled F(ab')2 fragments of monoclonal antibody OC125 which was reactive with CA125 antigen. Labeling efficiency was 99.9% and colloid formation was not observed. Specific activity was 1.48 GBq (40 mCi)/mg antibody and 99mTc OC125 preparation was stable more than 3 hr after labeling. Percent binding to 10(7) cells of PC-9 cancer cells expressing CA125 antigen was 47.0% in 99mTc OC125 almost the same as that of 125I OC125, Tumor-to-blood ratio for 99mTc OC125 was 5.73 +/- 1.41 at 18 hr after intravenous injection, which was significantly higher than that for 125I OC125. The scintigram showed a good localization in nude mouse bearing PC-9 tumor cells, and the clinical use of 99mTc OC125 is promising for the imaging of ovarian cancer.