N digestibility and the dietary quality of a Vicia faba protein isolate as well as of two protein fractions from this isolate, acetylated and nonacetylated, were estimated in 15N-labelled adult rats. For reference, whole egg, casein and wheat were studied. The diets were fed at maintenance energy intake level. Daily metabolic fecal nitrogen estimated by an isotopic method was 5 mg/l00 g body weight in animals receiving whole egg, casein, or Vicia faba proteins. When wheat was included in the diet metabolic fecal nitrogen was increased to 140% of this value. True N digestibility of reference proteins was 91-92% and of Vicia faba proteins 95-97%. Protein quality for maintenance was high and similar for all proteins. Acetylation affected neither N digestibility nor protein quality under maintenance conditions.