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The principles of selection are reviewed as a basis for discussing selection for protein content of milk. The correlations among components of milk will cause correlated responses in all even when selection is for only one component. Selection for fractional composition of fat or protein would lead to increases in content of fat and protein, but the expected increases in total yields of fat and protein would be much less than if selection were for yield of milk, fat, or protein. Selection should be for milk, fat, and protein yield with relative economic emphasis determined by the net economic value of the components. Market prices less costs of production could be used for fat and protein yield and a negative transportation charge used for milk yield. Costs of testing for protein should be considered carefully before doing so since expected economic improvement including protein yield is nearly as great for milk and fat yield as a basis for selection as for milk, fat, and protein yield.