Abstract: Thiazole esters are suitable for detecting the presence of leukocytes in urine. Such thiazole esters are suitable for use in compositions, diagnostic devices, and methods for detecting the presence of leukocytes. A thiazole ester of the invention is of the formula: or a salt or solvated salt thereof, in which A is an N-blocked amino acid residue or N-blocked peptide chain, preferably an alanine residue or polyalanine chain; and R1 and R2 are each independently hydrogen, unsubstituted or substituted aryl, unsubstituted or substituted heteroaryl, unsubstituted or substituted alkyl, unsubstituted or substituted alkenyl, unsubstituted or substituted alkoxy, amino, unsubstituted or substituted acyl, halo, nitro, cyano, —SO3H, or hydroxy, with the proviso that R1 and R2 are not both hydrogen. In one embodiment, at least one of R1 and R2 is methoxy, ethoxy, propoxy, or butoxy. In still another embodiment, R1 is hydrogen and R2 is methoxy, ethoxy, propoxy, or butoxy.