Late-exponential-phase Penicillium chrysogenum mycelia grown in a complex medium possessed an intracellular iron concentration of 650 mumol/L (2.2 +/- 0.6 mumol per g mycelial dry mass). This iron reserve was sufficient to ensure growth and antibiotic production after transferring mycelia into a defined low-iron minimal medium. Although the addition of Fe3+ to the Fe-limited cultures increased significantly the intracellular iron levels the surplus iron did not influence the production of penicillin V. Supplements of purified major P. chrysogenum siderophores (coprogen and ferrichrome) into the fermentation media did not affect the beta-lactam production and intracellular iron level. Neither 150 nor 300 mumol/L extracellular Fe3+ concentrations disturbed the glutathione metabolism of the fungus, and increased the oxidative stress caused by 700 mmol/L H2O2. Nevertheless, when iron was applied in the FeII oxidation state the oxidative cell injuries caused by the peroxide were significantly enhanced.