The effect of marginal intakes of energy on the requirement and utilization of egg protein was evaluated in 46 Japanese young men. The subjects were given a standard diet for 1 week and then low protein diets for 2 weeks. These diets contained about 32, 64, and 80 mgN/kg with whole eggs as the protein source. In the first experiment with excess energy, the energy intakes of 31 subjects were kept constant during the 3 week experiment, the mean intakes being 48.2 +/- 1.5 kcal/kg. The body weight was affected by changing protein intakes while maintaining energy intakes at 48 kcal/kg. From regression analysis, the N requirement for apparent N equilibrium was estimated to be 82.0 +/- 8.0 mgN/kg, where NPU was calculated as 56. In the second experiment with submaintenance energy, 15 subjects received 40 kcal/kg. The N requirement was 124 +/- 21 mgN/kg, where NPU was calculated as 37. From these results and those of previous studies, it was concluded as follows: 1) N balance and NPU were remarkably affected by energy intake changed around maintenance level; and 2) the NPU for egg protein in young men for maintenance intakes of energy and N is about 50 to 55. For estimation of the protein requirement for Japanese adults, a correction factor of 100/55 (about 1.8) was used instead of 1.3 adopted by the 1973 FAO/WHO.