There is an inoculum size effect associated with the bactericidal potency of 4-quinolones. This phenomenon has previously been attributed to anaerobic conditions at high initial inoculum sizes. The level of anaerobicity present with Escherichia coli and Staphylococcus aureus at varying initial inoculum size was analysed by measuring dissolved oxygen concentrations and redox potentials. Such measurements confirmed that conditions at high initial inoculum sizes are more anaerobic than conditions at low initial inoculum sizes. The precise concentration of oxygen at which 4-quinolones are no longer able to kill bacteria could not be deduced by either method. The results suggest that extremely low oxygen levels are needed to antagonise 4-quinolone kill. This explains why very stringent conditions are required to render media anaerobic enough to prevent the bactericidal activity of 4-quinolones against bacteria at low initial inoculum sizes.