Preliminary experiments were performed to investigate the possibility of combining chemical blocking with microbiological assays in a method for determining terminal amino acids in peptides;A subtractive chromatographic procedure for determination of terminal amino acid residues in peptides is presented. Results are given for a total of nine dipeptides and one tripeptide. By employing selective cleavage of the treated tripeptide with dioxane-HCl the sequence of residues could be determined;The benzoyl derivatives of nine amino acids essential for L. arabinosus, of twelve amino acids essential for L. brevis , and one of ten amino acids essential for Strep. faecalis were tested for utilization by these micro-organisms. It was found that, under the conditions of the experiment, L. arabinosus could utilise only benzoyl-DL-leucine with efficiency, whereas activity so slight as to be questionable was noted for dibenzyol-L-leucine with efficiency, whereas activity so slight as to be questionable was noted for dibenzoyl-L-cystine. Only benzoyl-DL-methionine was available for Strep. faecalis. L. brevis used hippuric acid efficiently, while results for dibenzoyl-L-cystine were equivocal for this organism also. It should be noted, however, that only one strain of each species of bacteria was used, and the results obtained may apply only to these strains;Twelve benzoylated dipeptides and one benzoylated tripeptide containing leucine, phenylalanine, and valine residues were tested for availability to L. arabinosus. Only two, hippuryl-DL-leucine and benzoyl-DL-alanyl-DL-leucine were utilized, the microorganism seeming to show a requirement for leucine at the carboxyl end of the compound used. One other such compound, however benzoyl-DL-leucyl-DL-leucine A, gave little or no growth response;Three peptides were essayed for availability to L. arabinosus. One of these, DL-valyl-DL-leucine, was found to support growth in a leucine-free medium but not free medium. A possible explanation is presented.