The d- and l-isomers of the following derivatives of valine, leucine, and tyrosine were prepared: the formyl, phthalyl, and prolyl derivatives of valine and leucine; the methyl ester hydrochlorides of valine and leucine; the glycyl derivatives of leucine; benzoyl, 3-amino, and 3-nitro tyrosine; the ethyl esters of tyrosine and benzoyl tyrosine; and benzoyl tyrosylamide;Of these, the prolyl and phthalyl derivatives of d-leucine, d-valine, and l-valine; the methyl ester hydrochlorides of d- and l-valine; the 3-amino and 3-nitro derivatives of d-tyrosine; and benzoyl-d-tyrosine ethyl ester were new compounds and constants for them were reported;High levels of glycine were found to inhibit the growth of L. arabinosus. The relationship between this inhibition and similar results reported in the literature for Streptococcus lactis was discussed;High levels of d-leucine and d-valine were also found to inhibit the growth of L. arabinosus. The same levels of l-leucine, l-valine, l-alanine, and d-alanine were without appreciable effect. Possible correlations between these results and the nutritional requirements of the organism, the structure of the amino acids, and the amino acids obtained from gramicidin were discussed;Of the valine, leucine, and tyrosine derivatives made, none seemed to show antipodal specificity. 3-Amino-l-tyrosine, 3-amino- d-tyrosine, d- and l-leucine methyl esters, and the phthalyl derivatives of both d- and l-leucine and valine showed more activity than d-leucine, although none of the activities was very high.