In an attempt to develop water-soluble, potent inhibitors for chymotrypsin, structurally rigid dipeptides containing arginine were designed and synthesized. The dipeptide H-D-Arg-Phe-NHBzl inhibited chymotrypsin very strongly (Ki = 5.9 microM). The dipeptide with the inverse sequence, H-D-Phe-Arg-NHBzl, was also a moderate inhibitor for chymotrypsin with Ki of 240 microM. In spite of the presence of arginine in these dipeptides, they inhibited trypsin only weakly, indicating that they are highly specific for chymotrypsin. High resolution 1H-NMR (400-MHz) indicated that these dipeptides can make a strong intramolecular hydrophobic interaction between Arg-beta, gamma, delta-methylenes and Phe-phenyl, producing a rigid hydrophobic core which interacts with the chymotrypsin S2 site. Since these dipeptides are easily soluble in water, they are regarded as the sophisticated and effective inhibitors for chymotrypsin.