A quantitative structure-activity relationship (QSAR) has been formulated for the inhibition of glyoxalase I from yeast by 37 S-substituted glutathiones: log 1/C = 1.23 pi' + 1.20 MR4 - 0.67 I1 - 0.14 pi'2 + 1.85 C in this expression is the molar concentration of inhibitor producing 50% inhibition, pi' is the usual hydrophobic parameter modified for certain substituents, MR4 is the molar refractivity of certain p-phenyl substituents, and I1 is an indicator variable for those congeners with an acetylated alpha-amino group. This equation should be of help in the design of more effective inhibitors which may be of value in cancer chemotherapy.