The apparent oxygen affinity of cytochrome bd from Escherichia coli and Azotobacter vinelandii has been measured using oxymyoglobin as a sensitive monitor of oxygen concentration. In membrane preparations, the Km(O2) and respiratory rate varied with the nature of the primary substrate used (malate, lactate, reduced nicotinamide adenine dinucleotide (NADH), or ubiquinol-1). At maximum respiratory rates, the Km(O2) for cytochrome bd from A. vinelandii was 4.1 microM, approximately 2 times higher than the corresponding value for the E. coli enzyme. There were no significant differences between the Km(O2) values for membrane-bound and purified cytochrome bd from A. vinelandii when ubiquinol-1 was used as primary substrate. The kinetic parameters Km(O2) and Vmax provide a value of 2.8 x 10(8) M-1 s-1 for the bimolecular rate constant for oxygen reaction with the enzyme, suggesting that this reaction is diffusion-controlled. Kinetic analysis indicates a mechanism involving a ternary complex. A scheme for the reaction mechanism of cytochrome bd is proposed.