The quality of different protein sources having a wide spectrum of potential biological values has been assessed with multi-point and single-point assays on growing Sprague-Dawley rats weighing 60 g. Values obtained with multi-point assays, expressed both as absolute and relative values, were well correlated irrespective of the response parameter, i.e. body weight change, body water or body nitrogen (r = 0.980). Single-point assay values based on body nitrogen content were also well correlated with the multi-point assays, but some discrepancies were noted for what assays based on body weight change was concerned. The modification of body protein concentration, in particular of rats fed the protein-free diet, was the main cause of these discrepancies. The problem of reference protein, when protein quality was expressed as relative value, was also discussed.