Simultaneous measurement of viscosity and degree of proteolysis of κ-casein, α, in skim milk at three different concentrations of rennet and three different concentrations of casein showed that viscosity was a function of α until the former reached its minimum value. Then the relation between viscosity and α depended on the enzyme concentration. This is considered to be caused by casein micelles starting to aggregate at this point. The minimum in viscosity was found when α was between 0·6 and 0·7, indicating that casein micelles started aggregating at a lower degree of proteolysis than was believed earlier. The rate of gel firming was found to depend on the concentration of added rennet even after κ-casein was completely proteolysed. The results demonstrated that both proteolysis of κ-casein and aggregation of casein micelles must be taken into account when modelling the renneting reaction.