During the renneting process in the manufacture of Cheddar cheese a retardation of the time of coagulation has often been noticed though the acidity had reached the required strength before the rennet was added to the milk. The cause or causes of this retardation are of great technical importance as they lead to infection of the milk with consequent production of taints. As the two main factors of coagulation are (i) the acidity of the milk when renneted, and (ii) the calcium salts of the milk, we approached the inquiry from these two points and tried to answer the following questions:—