The synthesis, characterization, and kinetic study of two linear pentapeptides L-threonyl-L-alanyl-L-seryl-L-histidyl-L-aspartic acid (TASHD) and L-seryl-gamma-aminobutyryl-L-histidyl-gamma-aminobutyryl-L- aspartic acid (Ser-Gaba-His-Gaba-Asp) that were previously reported by J. C. Sheehan et al. (Cruickshank, P.; Sheehan, J. C. J. Am. Chem. Sec. 1963, 86, 2070-2071 and Sheehan, J. C.; Bennett, G. B.; Schneider, J. A. J. Aln. Chern. Sec. 1966, 88, 3455-3456) to be esterolytic catalysts modeling the active site of chymotrypsin is described. In contrast to these two reports, we observed no catalysis of ester hydrolysis for substrates p-nitrophenyl acetate (p-NPA), N-methoxycarbonyl- L-and-D-phenylalanine-p-nitrophenyl esters using the original conditions reported. In order to probe our conflicting results, accurate measurement of second order rate constants for the hydrolysis of each of these three p-nitrophenyl esters was carried out. The second order rate constants for these hydrolysis reactions were an order of magnitude lower than those originally reported for peptide Ser-Gaba-His-Gaba-Asp and three times lower than those reported for peptide TASHD. A very small difference in the rates of hydrolysis of N-methoxycarbonyl-L-and-D-phenylalanine-p-nitrophenyl esters in the presence of peptides TASHD and Ser-Gaba-His-Gaba-Asp was observed. Kinetic studies of the hydrolysis of p-NPA in the presence of peptide TASHD using a variety of concentrations of substrate and peptide are reported. A series of measurements of initial rates of hydrolysis of p-NPA in the presence of peptide TASHD provided no evidence for saturation kinetics. Treatment of peptide TASHD with the serine-protease inhibitor diisopropylfluorophosphate (DFP) produced no retardation in the initial rates of hydrolysis of p-NPA. The second order rate constants for the two peptides fell on the Bronsted line for a variety of substituted imidazoles from the literature. Our interpretation of these findings is that the peptides are behaving as simple imidazole catalysts for the hydrolysis of p-NPA and that there is no evidence for the involvement of the serine residue side chain during catalysis or any substrate binding by these peptides.