The present invention discloses a fermentative process for the production of penicillin G or V or cephalosporin G or V derivatives wherein certain derivatives of phenylbutyric acid are used as a side chain precursor. In said phenylbutyric acid derivatives, the acyl chain is extended by pairs of carbon atoms and certain substituents are present at the φ- and/or φ-1 position. The corresponding phenoxybutyric acid derivatives are used to produce penicillin V or cephalosporin V derivatives. Optionally, the penicillin or cephalosporin G or V compounds are deacylated to produce their deacylated counterparts.