Eighteen Holstein cows were used for three consecutive lactations to compare 13, 15, and 17% protein rations. Cows were assigned to six 3×3 Latin squares. Each cow was fed individually and received a different protein concentration for an entire lactation. Alfalfa hay and the designated concentrate were fed ad libitum twice daily. Cows receiving the 15 or 17% protein rations produced more milk, fat, protein, and milk solids-not-fat than those receiving the 13% protein ration. Percentages of milk components did not differ among rations. Cows receiving the 13% protein ration consumed less concentrate than those receiving the 15% or 17% rations. Hay, total dry matter intake, body weight, services per conception, or days cows were open were not different among dietary protein concentrations.The 17% protein ration was the least efficient for converting feed protein to milk protein. No other measure differed between the 15 and 17% protein rations; thus, 15% protein as recommended by the National Research Council appears adequate for cows producing to 29kg milk per day. We concluded that a ration containing 13% protein was insufficient for cows producing to 29kg of milk per day. Moreover, 13% protein also was inadequate for cows in mid- and late lactation, apparently because they had received insufficient protein during early lactation.