An improved method for the microbiological assay of available amino acids in proteins using Tetrahymena pyriformis.

: 1. A standard microbiological procedure for the assay of available amino acids in proteins using Tetrahymena pyriformis has been modified by evaluating cell growth from the tetrahymanol content of the cultures by gas-liquid chromatography (GLC). 2. This modification improves the precision of the method compared with cell-counting techniques and provided automatic injection facilities are available for the GLC, the procedure is not excessively time-consuming. 3. It has been confirmed that enzymic predigestion is essential for all except the most soluble proteins and a satisfactory method using pronase has been incorporated into the microbiological procedure. 4. After enzymic digestion of the sample, growth of Tetrahymena with many proteins can be measured turbidimetrically, but particulate matter still persists with a proportion of samples, particularly those of plant origin, and the GLC method is therefore more widely applicable. It is also more precise. 5. Good agreement has been obtained between a chick bioassay and the modified Tetrahymena assay for available lysine in eight different proteins, including heat-damaged fish meals. 6. The modified procedure appeared to give good results for available tryptophan and methionine. 7. When cystine was omitted from the standard culture medium it was possible to test for available methionine plus cystine, but further work is required to assess the reliability of this particular assay.