1. Results from metabolic balance studies in young adult men were examined for the relationships between the concentration of serum urea (SUN) and the efficiency of dietary nitrogen utilization. A total of 231 separate balance periods were included in the analysis. Net protein utilization (NPU) was used as an index of dietary protein quality.
2. Protein intake ranged from 0·27–0·73 g/kg body-weight per d in the various experiments. There was a highly significant negative correlation between NPU and urea levels. The relationship was: NPU = 1·23–0·029 × SUN (mmol/l) (r = −0·89).
3. These results are considered in relation to the evaluation of protein quality in adult humans.