A simple system was developed, consisting of a solution of β-glycerophosphate, β-lactoglobulin and magnesium ions, in which alkaline phosphatase isolated from milk became strongly reactivated following heat treatment for 45 sec in boiling water. From 10 to 30% of the original enzyme activity reappeared after incubation at 37°C. Variations in the components of the system, and factors affecting it, were studied. Salts of Hg, Zn and Cd inhibited reactivation at low concentrations.

Milk which, after the same heat treatment, became reactivated to a much smaller extent (about 1%), was found to contain a dialysable, heat labile inhibitor whose presence is thought to be largely responsible for the low level of reactivation, though other factors, e.g. the suboptimal concentration of phosphate esters must be considered as contributory causes.

Milk contains also a non-dialysable, heat stable activator of the reactivation process, capable of replacing β-lactoglobulin in the simple system and active at very low concentration, e.g. 0·2% (v/v).