A study was conducted to estimate protein quality in 4 days. Protozoan bioassays were combined with in vitro protein digestibility procedures. Data are included for 14 protein samples having a PER range of 0.68 to 2.83. In vitro protein digestibility values were in close agreement with rat apparent protein digestibility values. Two equations for predicting PER were derived from the in vitro digestibility and Tetrahymena bioassay data. Both equations have high multiple correlation coefficients (r=0.92 r=0.93). Rat PER data and computed PER data are listed. The standard deviation of the differences between the observed and computed PER is 0.28 PER (Equation 1) and 0.26 PER (Equation 2). This is compared to a standard deviation of 0.33 PER for the authors' observations of the variation in the rat bioassay.