Studies were made in adult rats using an isotopic 15N method. Whole egg, wheat, casein and Viciafaba protein isolate served as reference proteins. The diets were isonitrogenous and feed intake was adjusted to the maintenance energy level. Variations in true N digestibility were small (91.8–96.1%). The amount of metabolic fecal nitrogen in animals receiving the diet containing wheat was 9.4 mg/l00 g body weight compared with 4.9–5.8 mg in the other diets. The differences in nutritional quality for maintenance between proteins of plant and animal origin were small. However, spinning (alkaline treatment) and additional cross-linking with aluminum ions or dialdehyde starch affected the nutritional quality for maintenance.