In this letter we report the synthesis and biochemical evaluation of selective, irreversible diphenyl phosphonate inhibitors for urokinase plasminogen activator (uPA). A diphenyl phosphonate group was introduced on the substratelike peptide Z-d-Ser-Ala-Arg, and modification of the guanidine side chain was investigated. A guanylated benzyl group appeared the most promising side chain modification. A k(app) value in the 10(3) M(-1) s(-1) range for uPA was obtained, together with a selectivity index higher than 240 toward other trypsin-like proteases such as tPA, thrombin, plasmin, and FXa.