Prostate-specific antigen (PSA) is a serine protease produced at high levels by normal and malignant prostate epithelial cells that is used extensively as a biomarker in the clinical management of prostate cancer. To better understand PSA's role in prostate cancer progression, we prepared a library of peptidyl boronic acid-based inhibitors. To enhance selectivity for PSA vs other serine proteases, we modified the P1 site of the inhibitors to incorporate a bromopropylglycine group. This allowed the inhibitors to participate in halogen bond formation with the serine found at the bottom of the specificity pocket. The best of these Ahx-FSQn(boro)Bpg had PSA Ki of 72 nM and chymotrypsin Ki of 580 nM. In vivo studies using PSA-producing xenografts demonstrated that candidate inhibitors had minimal effect on growth but significantly altered serum levels of PSA. Biodistribution of (125)I labeled peptides showed low levels of uptake into tumors compared to other normal tissues.