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Abstract: Rationale: Ovarian cancer is the most lethal malignancy of the female reproductive system. ~80% of ovarian cancers are epithelial in origin and commonly classified as Epithelial Ovarian Cancer (EOC). This malignancy is characterized by an overexpression of Cancer Antigen-125 (CA125) – a cell surface mucinous glycoprotein that serves as a USFDA approved ovarian tumor associated antigen. However, the early detection of EOC is plagued by its asymptomatic nature of progression and the limitations of currently used front-line diagnostic tools such as immunoassays that are capable of detecting CA125 as a shed antigen in the serum of patients presenting in the clinic with pelvic masses suspected for ovarian cancer. At present, there is no technique available for the in vivo evaluation of CA125 expression in malignant tissues, which has been shown to be an early event in the recurrence of epithelial ovarian cancer. Hypothesis: CA125 is a suitable target for the molecular imaging of epithelial ovarian cancer. Methods: An immuno-PET strategy was devised to employ CA125-targeted monoclonal antibody (MAb B43.13) and its derivative single chain Fragment variable (scFv) as molecular probes for imaging in vivo expression of CA125 via positron emission tomography. Anti-CA125 MAb and scFv were prepared and functionally characterized for their targeting capabilities prior to and post radiolabeling them for use in immuno-PET imaging. In separate applications of this strategy, we employed three different PET-radionuclides – 18F, 64Cu and 89Zr with suitable versions of the antibody molecule as targeting vectors to carry out same-day, next day or later time point in vivo imaging of EOC in subcutaneously xenografted mice models. Results: Biochemical methods of analysis including immunofluorescence, flow cytometry and immunoblotting revealed highly specific binding of the antibody vectors to CA125-positive NIH:OVCAR-3 cells with no binding to CA125-negative SKOV3 cells. Radiolabeled versions of the anti-CA125 MAb and scFv were obtained with high specific activity and both radioimmunoconjugate vectors demonstrated highly selective binding to NIH:OVCAR-3 cells and virtually no binding to SKOV3 cells. In vivo radiopharmacological evaluation of the anti-CA125 radioimmunoconjugate vectors in xenograft mice models provided consistently high absolute uptake values in NIH:OVCAR-3 tumors and minimal uptake in SKOV3 tumors. Results from small animal PET imaging were confirmed by ex vivo digital autoradiography, immunofluorescence and immunohistochemistry. Conclusions: CA125 is a suitable target for non-invasive imaging of epithelial ovarian cancer. Radiolabeling of anti-CA125 MAb and scFv with positron emitting radionuclides did not compromise their immunoreactivity to the target antigen. Both the antibody-based radioimmunoconjugates presented targeted tumor accumulation and an expected in vivo biological clearance profile. This renders them as potential immuno-PET probes for targeted in vivo molecular imaging of CA125 in Epithelial Ovarioan Cancer.