Compounds of formula (II): ##STR1## in which Y represents ##STR2## and R.sub.0 represents a C.sub.2-20 alkyl, C.sub.3-8 cycloalkyl, C.sub.4-20 alkenyl, arakyl, cycloalkylalkyl, heterocyclyl or heterocyclylalkyl group, which is substituted by a hydroxyiminohydrazono- or semicarbazono group, have activity against human and veterinary bacteria and mycoplasma. They may be produced by conventional methods and are used in conventional formulations.