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Forty-six infant rhesus monkeys of both sexes were fed a for­mula containing 3.35%, 6.7%, or 13.4% protein as casein. Their weight gain and food consumption were compared with those of 53 monkeys fed a commercial formula simulating human milk. All groups passed through a three-day weight-loss stage and a transitional stage lasting about one week, after which they entered a rather permanent growth stage. Males ate more than the females did, but only because they weighed more. The lowest- protein diet had no significant deleterious effect on weight gain during the transition phase, and it had no effect on the females’ gain during the growth stage. Males fed the lowest protein diet, in contrast, gained little during the growth stage. The reduction in weight gain seems due to a loss in food efficiency, not to a reduction in consumption.