Abstract A simple, rapid, and sensitive method is described for the semiquantitative determination of contaminating amino acids in amino acid preparations. The dry amino acid powders were assayed directly by microbiological plate assay methods for amino acids. By extrapolation of standard lines, contaminations down to the 0.001% level on a weight basis of the test material could be estimated for the following 8 amino acids: arginine, cyst(e)ine, histidine, lysine, methionine, threonine, tryptophan, and tyrosine. Quantitative microbiological plate and tube assays were also carried out with highly concentrated and neutralized amino acid solutions, permitting the assay of the same amino acid contaminations down to the 0.01% level. The results are given of purity tests on over 250 different amino acid batches from 8 commercial suppliers of the common 20 amino acids. The purity of the investigated amino acids with respect to contamination by the 8 aforementioned amino acids was very high. Only 25 amino acid batches had a contamination of 0.10% and above.