Abstract: Novel cephem derivatives represented by the general formula ##STR1## in which the Acyl substituent is a group of the formula ##STR2## wherein Ar is a lipophilic optionally substituted phenyl, naphthyl or pyridyl group; R.sup.1 represents either the residue of a heterocyclic amino acid or a C.sub.2 -C.sub.10 alkyl group substituted by both a carboxyl group and a group of the formula --NR.sup.9 R.sup.10 or ##STR3## in which R.sup.9 and R.sup.10 are each independently hydrogen or C.sub.1 -C.sub.6 alkyl, said C.sub.2 -C.sub.10 alkyl group being optionally interrupted by one or more nitrogen atoms or carbonyl groups, and R.sup.2 and R.sup.3 are each independently hydrogen, alkyl or aminoalkyl are gram-positive antibacterial agents, especially useful in the treatment of diseases caused by methicillin-resistant Staphylococcus aureus (also referred to below as MRSA or methicillin-resistant S. aureus).