1. The concentration of 2-aminoethylphosphonic acid (AEP) in 96 h cultures of Tetrahymena pyriformis W was studied in order to apply it as an indicator in the assay of the relative nutritive value (RNV; protozoa population with test protein:protozoa population with whole-egg powder) of protein. Foodstuffs and food mixtures of different protein contents and qualities were used as test samples. 2. RNV values based on AEP determination (RNVAEP) were compared with corresponding values calculated from protozoa counts (RNVpc), as well as with biological value (BV) and net protein utilization (NPU) of the same proteins assayed on rats. 3. Both for foodstuffs and food mixtures highly significant correlations were found between RNVAEP and RNVpc, RNVAEP and both BV and NPU, and RNVpc and both BV and NPU. 4. AEP content in the protozoal suspension was preferred to cell count as a measure of growth response, since it took into account large differences in cell dimensions that were observed between cultures grown with different test proteins.