Protein quality of feeding-stuffs. 7. Collaborative studies on the microbiological assay of available amino acids.

1. Twenty-two samples of high-protein feeding-stuffs, sixteen of them fish meals, were used in a collaborative study of the precision and the limits of discrimination of the Streptococcus zymogenes assay procedure, as applied to the estimation of available methionine, tryptophan and isoleucine contents. 2. All the participating laboratories ranked the test samples in much the same sequence with respect to content for all three amino acids. There were apparently systematic differences between laboratories which impaired the precision of some of the estimates, and these were greatly reduced by including common reference sample in the tests as an auxilliary standard. 3. Values for available methionine content for eleven test samples were highly correlated (r 0.86) and quantitatively similar to those obtained for chick growth assays, but those for available tryptophan content were markedly lower and were probably in error.