The overall vegetable and animal protein combinations of the diet in Northern Mexico were determined through a dietary population survey. Vegetable sources made up 45% and animal protein was 55% (45V/55A). Further combinations of up to 100% vegetable protein dietary mixtures (100V) were studied to test the sensibility of the variations on protein requirements of pre-school, school children and adults. Diets were analyzed for amino acid composition and in vivo protein digestibility in rats to estimate true protein requirements according to FAO/WHO/UNU (1985). The effect on the preschool group showed the widest variation with 1.46 g/kg/day in the 45V/55A to 2.63 in the 100V. For the school-aged children and adults the variations were 1.15-1.79 and 0.94- 0.84 g/kg/day respectively.