Mycobacterium neoaurum was grown with a range of iron concentrations from 0.01 to 4.0 µg/ml. Synthesis of the extracellular siderophore, exochelin, the intracellular iron storage compound, mycobactin and the iron-repressible envelope proteins were co-ordinately expressed. All three components of the iron transport system were synthesized when low amounts of iron (0.01 to 0.2 µg/ml) were added to the medium and were repressed when the iron concentration was increased to 0.5 µg/ml and above. These results re-inforce the conclusion that the iron-regulated proteins do fulfil an essential function in iron metabolism.