Abstract: The invention provides novel antibiotic compounds which are 6.beta.-acylamidopenam-3-carboxylic acids and non-toxic derivatives thereof, characterized in that the acylamido group has the structure ##EQU1## WHERE R is a hydrogen atom or an organic group and R.sup.a is a hydrogen atom or an acyl group. The compounds are syn isomers or exist as mixtures containing at least 75% of the syn isomer. These antibiotic compounds possess high antibacterial activity against a range of gram positive and gram negative organisms coupled with particularly high stability to .beta.-lactamases produced by various gram negative organisms. The invention is also concerned with the administration of the compounds.

Abstract: The invention provides novel antibiotic compounds which are 6.beta.-acylamidopenam-3-carboxylic acids, and non-toxic derivatives thereof, characterized in that the acylamido group has the structure ##EQU1## WHERE R is a hydrogen atom or an organic group and R.sup.a is an etherifying monovalent organic group linked to the oxygen atom through a carbon atom. The compounds may be either syn or anti isomers or may exist as mixtures e.g. containing at least 75 percent of one isomer. These antibiotic compounds possess high antibacterial activity against a range of gram positive organisms including penicillinase - producing staphylococci coupled with high activity against strains of the gram-negative organism Haemophilus influenzae. The invention is also concerned with the administration of the compounds.