Cephem derivatives of the general formula ##STR1## in which the R.sub.2 O group is in the syn-position, a process for their manufacture and pharmaceutical formulations which are active against bacterial infections and contain these compounds.
A series of fused bicyclic 2-aminothiazolyl compounds were synthesized and evaluated for their synergistic effects with polymyxin B (PB) against Klebsiella pneumoniae (SIPI-KPN-1712).