Better tumor markers are needed for early diagnosis and staging of prostate cancer, and for monitoring therapeutic response than the currently used prostate specific antigen (PSA). Prostate specific membrane antigen (PSMA) is highly expressed on the surface of prostatic epithelial cells making it a good target for prostate cancer. In this study, mAb 3C6, specific for the extracellular epitope of PSMA, was evaluated both in vitro and in vivo for PSMA-targeting. Immunoreactivity and specificity of mAb 3C6 was evaluated by flow cytometry using prostate cell lines expressing PSMA such as LNCaP and 22Rv1 and a cell line, DU145, that expresses very little PSMA. 3C6 was conjugated with the acyclic CHX-A” DTPA chelate, radiolabeled with 111In, and its in vitro and in vivo properties were assessed. The biodistribution of the radioimmunoconjugate evaluated in athymic mice bearing xenografts of three human prostate carcinoma cell lines shows high uptake after 72 hr in LNCaP tumors (%ID/g 22.93 + 6.32) and 22Rv1 (%ID/g 10.44 + 2.32) in contrast to low uptake by the DU145 tumors (%ID/g 4.27 + 0.37). Planar γ-scintigraphic images obtained for xenografted tumor bearing mice demonstrated targeting for PSMA positive tumors suggesting possible applications in imaging and for targeted radiation therapy.