1. Three spray-dried separated milk powders with moisture contents of 2·9, 4·7 and 7·3% (3 hr. air oven) or 3·0, 5·0 and 7·6% (20 hr. vacuum oven) were packed in air and in almost pure nitrogen, in gas-tight cans, and stored at 20·0, 28·5 and 37° C, for a period of nearly 2 years.
2. The powders were examined at intervals for palatability, colour, pH, equilibrium relative humidity (indicating crystallization of the lactose), conversion of β-lactose to α-lactose hydrate, decrease in total soluble lactose, absorption of oxygen, production of carbon dioxide, solubility in water at 20 and at 50 or 60° C, changes in the distribution of soluble nitrogen, reducing power towards potassium ferricyanide, base-binding capacity, formol titration, weight of the undialysable fraction, free amino-nitrogen by the Van Slyke method, sugar attached to the protein, and bacterial content.