The method of Osborne, Mendel and Ferry for the measurement of protein efficiency ratios (grams gain in body weight per gram of protein consumed) has been adapted for use with mice. The importance of establishing standardized conditions with regard to the pre-test treatment of the test animals, the duration of the test period, and the level of test protein in the diet has been demonstrated. The advantages of the use of the mouse as compared with the rat are: smaller animal size, lower food consumption, and shorter test periods.