A novel thioalkylthio cephalosporin antibiotic compound of formula I: ##STR1## wherein Acyl is C.sub.1 -C.sub.12 acyl; Het is optionally substituted monocyclic heteroaromatic group containing one or more hetero atoms; R.sup.1 is a single bond or C.sub.1 -C.sub.4 alkylene; R.sub.2 is a straight or branched C.sub.1 -C.sub.4 alkylene; X is a sulfur atom or sulioxide group; and Y is a hydrogen atom or methoxy group, or a pharmaceutically acceptable salt or an amino-, carboxy- and/or hydroxy-protected derivative thereof, a formulation containing the same and a method for treating bacterial infections.