Prostate-specific membrane antigen (PSMA) has been identified as a diagnostic and therapeutic target for prostate cancer. (S)-2-[3-[(R)-1-Carboxy-2-mercaptoethyl]ureido-pentanedioic acid (Cys-CO-Glu) were used to design novel PSMA targeting probes by nucleophilic conjugate addition between cysteine and maleimide based reagents. 3 ([(123)I]IGLCE) was synthesized by this strategy and showed high affinity for PSMA. Results of binding inhibition assays of these derivatives suggested the importance of an aromatic group and succinimide moiety for high affinity. [(123)I]3 was evaluated in vivo with PSMA positive LNCaP and PSMA negative PC-3 human prostate cancer xenograft bearing mice. [(125)I]3 accumulated in LNCaP tumors but not in PC-3 tumors, and the accumulation was inhibited by 2-(phosphonomethyl)pentanedioic acid (2-PMPA). Use of [(123)I]3 provided positive images of LNCaP tumors in single photon emission tomography scans. These results warrant further evaluation of [(123)I]3 and its derivatives as radiolabeled probes for the diagnosis of prostate cancer.