Carbon monoxide (CO) is a classical respiratory inhibitor, but CO-releasing molecules (CO-RMs) have therapeutic value, increasing phagocytosis, and reducing sepsis-induced lethality. CORM-3, Ru(CO)(3) Cl(glycinate), a ruthenium-based carbonyl that liberates CO under physiological conditions, has previously been shown to inhibit bacterial growth and respiration, even at high concentrations of oxygen. Here, we report the effects of CORM-3 on the microaerophilic foodborne pathogen Campylobacter jejuni. Even at CO-RM (i.e., CO) concentrations that exceed dissolved oxygen levels, CORM-3 does not inhibit microaerobic growth. This insensitivity is not due to failure of CORM-3 to penetrate cells, as revealed by assay with extracellular myoglobin and by the ability of CO from externally added CORM-3 to bind intracellular membrane-associated respiratory oxidases. Even at almost 200 μ M oxygen, CORM-3 inhibits formate-dependent respiration and leads to generation of hydrogen peroxide. This work shows that CO-RMs have valuable properties as antimicrobial agents; however, growth inhibition does not always accompany inhibition of respiration, even when ambient oxygen concentrations are low.