The purpose of this investigation was 1) to determine the apparent and true digestibilities in mice of nitrogen (N) and amino acids (AA) in commercially refined protein sources, and 2) to estimate metabolic fecal nitrogen (MFN) excretion. Each protein source was fed to adult female mice at 15 and 30% of a purified diet. A protein-free diet was included. Nutrient digestibility was determined by a fecal balance method with chromic oxide as a marker. Apparent digestibility of N in each protein source reflected the dietary N concentration and AA digestibility showed a similar trend. True digestibility (TD) of N was 97-98% for Animal Nutrition Research Council reference protein, casein, gelatin and gliadin, 94-96% for lactalbumin and soy protein and 81% for zein. TD of AA generally reflected TD of N. MFN excretion (g/100 g dry matter consumed) was 0.146 when the protein-free diet was fed and 0.151 by extrapolation to zero N intake. It was concluded that when commercially refined proteins are fed to mice, MFN excretion can be determined by either method, but this may not apply when the protein source contains antinutritive substances.