Cephalosporin derivatives having a 3-position substituent of the formula I are described: ##STR1## wherein R.sup.1 is hydrogen, alkenyl or optionally substituted alkyl, Het us a 5- or 6-membered heterocyclic ring bonded via a carbon atom to the amide linkage, wherein Het is selected from a group of the formulae II-III: ##STR2## wherein A is CH or a nitrogen atom; B is oxygen, sulphur or a group NR.sup.4 ; one or two of D, E, F and G are nitrogen atoms and the remainder are CH groups: or Het is a pyrazinone, pyridinone, pyridazinone or pyrimidinone ring, or is a thione equivalent of such a ring, said rings having a substituent R.sup.4 on one nitrogen atom, or is pyranone, or pyranthione; the ring Het being fused by any adjacent carbon atoms to the benzene ring; and Het being attached to the --CH.sub.2 NR.sup.1 CO-- group via a carbon atom;R.sup.2 is hydroxy or an in vivo hydrolyzable ester thereof;R.sup.3 is ortho to R.sup.2 and is hydroxy or an in vivo hydrolyzable ester thereof; and R.sup.4 has various values. The use of such compounds as antibacterial agents is described as are processes for their preparation and intermediates therefor.