Abstract: The invention deals with novel peptides useful for the therapeutic treatment of infectious diseases caused by pathogenic microorganisms. Included is the peptide FR900156 having the structure: ##STR1## as well as the peptides of the structure ##STR2## wherein R.sup.1 is acyl;R.sub.b.sup.1 is hydrogen, methyl, isopropyl, hydroxymethyl, protected hydroxymethyl or benzyl;R.sup.2 is hydrogen, carboxy, protected carboxy, or a group of the formula: ##STR3## wherein R.sub.a.sup.2 is mono- or di-carboxy lower alkyl or ar(carboxy) lower alkyl whose aryl moiety may be substituted by hydroxy,R.sub.b.sup.2 is hydrogen or lower alkyl;R.sup.p and R.sup.q are each hydrogen, carboxy, protected carboxy, with proviso that when one of R.sup.2 and R.sup.q is hydrogen, then the other is carboxy or protected carboxy;R.sup.r is hydrogen or amino protective group; m is an integer 1 to 3, and n is 1 .Iadd.provided that when R.sup.1 is benzyloxycarbonyl R.sub.5.sup.1 is methyl, m is an integer 2 and n is an integer 1, then R.sup.