A range of analogues of parabactin A 4 and agrobactin A 5 have been synthesised. All these comounds were able to chelate iron, and to promote the growth of iron-starved Paracoccus denitrificans to varying degrees. A degree of flexibilty in the siderophore receptor system is thus demonstrated which may be exploitable for the transport of new siderophore - antibiotic conjugates.