Abstract: The present invention is concerned with methods of potentiating an antibiotic. The invention also includes compositions of an antibiotic and a peptide having units of the formula:(a) (A).sub.n wherein A is Lysine or Arginine and n is an integer with a minimum value of 7.(b) (AB).sub.m wherein A is Lysine or Arginine and B is a hydrophobic amino acid selected from the group consisting of Valine, Leucine, Isoleucine, Tyrosine, Phenylalanine and Tryptophan; m is an integer with a minimum value of 3; and(c) (ABC).sub.p wherein A is a cationic amino acid which is Lysine or Arginine; B and C are hydrophobic amino acids which may be the same or different and are selected from the group consisting of Valine, Leucine, Isoleucine, Tyrosine, Phenylalanine and Tryptophan; p is an integer with a minimum value of 2. The compositions have potentiated antibiotic activity.