Alkylation of cephem-4-carbonyl chlorides at the sulphide or sulphone oxidation level with Grignard reagents, stannanes and cuprates is described. Radical or ionic bromination of the 3-methylcephem-4-ketone 8, followed by displacement with heterocyclic thiols, provides an entry to 3'-and 2-thiosubstituted derivatives, which are new potent inhibitors of HLE.