The invention relates to novel peptides and their pharmaceutically acceptable salts, particularly useful for pharmaceutical purposes such as in the therapeutic treatment of infectious diseases caused by pathogenic microorganisms. The compounds have the structure ##STR1## wherein R.sup.1 is hydrogen or acyl;R.sup.2 is carboxy or protected carboxy or a group of the formula: --COHN--R.sub.a.sup.2 wherein R.sub.a.sup.2 is carboxy (lower) alkyl or protected carboxy (lower) alkyl;R.sup.3 is carboxy, protected carboxy, lower alkyl, hydroxyphenyl, carbamoyl or a group of the formula: ##STR2## wherein R.sub.a.sup.3 is hydrogen, amino, protected amino or acylamino, R.sub.b.sup.3 is carboxy or protected carboxy,R.sup. is carboxy, protected carboxy, carbamoyl, carboxy (lower) alkyl or protected carboxy (lower) alkyl;l is an integer 2,m is an integer 0 to 2 andn is an integer 1, provided that, whenR.sup.1 is hydrogen or acyl,R.sup.2 is carboxy, protected carboxy or a group of the formula: --CONHR.sub.a.sup.2 wherein R.sub.a.sup.2 is carboxy (lower) alkyl or protected carboxy (lower) alkyl,R.sup.P is carboxy, or protected carboxy, m is an integer 2, thenR.sup.3 is carbamoyl, lower alkyl or hydroxyphenyl or a group of the formula: ##STR3## wherein R.sub.a.sup.3 is hydrogen or acylamino and R.sub.b.sup.3 is carboxy or protected carboxy, and whenR.sup.P is carbamoyl, thenR.sup.1 is acyl.