A method of assay for histidine and lysine in protein hydrolyzates employing Leuconostoc mesenteroides as test organism is descr. The avg. values (expressed as %; moisture-free basis) obtained for the lysine and histidine contents, respectively, of 6 purified proteins are as follows: casein 7.5, 3.2; gelatin 3.87, 0.83; ovalbumin 5.6, 2.4; B-lactoglobulin 10.5, 1.73; horse Hb 8.4, 8.2; silk fibroin 0.58, 0.41. Also, a method of assay for lysine, arginine, and valine using Streptococcus faecalis as test organism is descr. Avg. values (moisture-free basis) obtained by this method are: casein[long dash]7.6% lysine, 3.71% arginine. 6.91% valine; gelatin[long dash]5.2% lysine, 8.77% arginine, 2.6% valine; ovalbumin[long dash]6.0% lysine, 5.7% arginine, 7.36% valine; [beta]-lactoglobulin[long dash]10.4% lysine, 2.88% arginine, 5.91% valine; horse hemoglobin[long dash]8.66% lysine, 2.8% arginine, 9.4% valine; silk fibroin[long dash]0.90% lysine, 0.99% arginine, 3.41% valine. With few exceptions, the data reported agree closely with those obtained by previous investigators employing microbiologic methods.