Compositions having antibiotic properties containing as antibiotic potentiators peptides of the formula ##STR1## wherein R.sup.1 represents a hydrogen atom or a lower alkyl, lower cycloalkyl, (lower cycloalkyl)-(lower alkyl), aryl or aryl(lower alkyl) group (said group being optionally substituted as the case may require by one or more amino, hydroxy, thio, methylthio, carboxy or guanidino groups so as to form the characterizing group of a naturally occurring L .alpha.-amino acid);R.sup.2 and R.sup.3 each represent the characterizing group of an .alpha.-amino acid of the type normally found in proteins with the proviso that R.sup.3 cannot represent a hydrogen atom when n is zero and R.sup.1 is a hydrogen atom or the phenyl group; R.sup.4 represents a hydroxy or methyl group; n stands for zero, 1, 2 or 3; and single asterisks denote that the configuration at the carbon atom so-marked is L when R.sup.2 or R.sup.3 as the case may be represents other than a hydrogen atom; and the double asterisk denotes that, when R.sup.1 represents other than a hydrogen atom, the configuration at the carbon atom so-marked is (R) (as hereinafter defined), and an antibiotic are disclosed.