True kinetics of kappa-casein hydrolysis with chymosin immobilized in Ca-alginate was investigated in pure kappa-casein and reconstituted milk solutions at 7, 22, and 37 degrees C. The tore Michaelis-Menten constant (K-M,K-T) was estimated to be 0.263, 0.154, and 0.133 mu mol ml(-1) at 7, 22, and 37 degrees C in the milk solution, respectively. It was estimated to be 0.031 mu mol ml(-1) on average in the K-casein solution within the given temperature range. The tote reaction rate constant (k(3,T)) was estimated to be 14.7, 21.6, and 46.5 s(-1) in the milk solution and 43.7, 75.2, and 146.4 s(-1) in the kappa-casein solution at 7, 22, and 37 degrees C, respectively. The effect of temperature on k(3,T) was expressed by the Arrhenius equation. The activation energy (E-a) was estimated to be 6,610 x 10(-6) cal mu mol(-1) in the milk solution and 6,993 x 10(-6) cal mu mol(-1) in the kappa-casein solution.