Piperidine derivatives and pharmaceutically acceptable derivatives thereof useful in methods of treatment of bacterial infections in mammals, particularly in man. The piperidine derivatives include compounds of formula (I): ##STR00001## wherein R.sup.A, AB, n, R.sup.3, R.sup.2 and R.sup.4 are defined herein. For example, R.sup.A is an optionally substituted bicyclic carbocyclic or heterocyclic ring system, and R.sup.4 is a group U-R.sup.5 in which R.sup.5 is an optionally substituted bicyclic heterocyclic ring system as defined herein (e.g. optionally substituted pyrido [3,2, -b][1,4] thiazin-yl).