The heats of reaction of trypsin with soybean (Kunitz), ovomucoid, and lima bean inhibitors have been measured at pH 5.0 and 10’ and 25”. All the reactions were endothermic, with AH ranging from 8.6 Cal per mole for the lima bean inhibitor to 15.3 Cal per mole for the soybean inhibitor. Equilibrium constants for the association were calculated from the dependence of residual tryptic activity up...