Twenty-nine adult cebus monkeys (18 males and 11 females) were used in long-term feeding experiments designed to study the protein requirements of this species. By feeding an otherwise adequate diet containing graded levels of lactalbumin, it was shown that diets containing 7.5% of the calories as protein were necessary for long-term weight maintenance. This estimate is compared to data obtained with young growing cebus monkeys in which 7% of the calories was sufficient for maximum growth, although it must be emphasized that due to its greater caloric intake/kg body weight, the protein intake/kg body weight of the younger animal is higher. Whereas a diet containing 9.34% protein supplied by dried bread crumbs (bread diet) was insufficient for weight maintenance of the adults, additions of 4 g lysine/kg bread crumbs and 1.5 g each methionine and threonine/kg bread crumbs produced a diet indistinguishable from the control diet (4.7% bread protein + 4.7% lactalbumin). When wheat gluten was added to the bread diet elevating the protein content to 16.2% of the calories, the amount of lysine necessary to improve the diet to weight maintenance levels increased when expressed/100 dietary kcal as compared to the bread diet alone, although the amounts in both diets were similar when expressed/g of dietary protein.