A method for on-line, continuous measurement of the oxygen consumption rate (Io(2)) by mammalian cells on a microcarrier was developed and its reliability investigated. Utilizing the periodic dissolved oxygen (DO) fluctuation in the normal on-off control of DO, on-line, continuous measurement of Io(2) was carried out in which real-time estimation of the DO saturation concentration was made by measuring the gas-phase pressure and the gas-phase oxygen concentration. It was found that the continuously measured Io(2) value was quantitatively exact and could be applied commercially using the oxidation reaction of glucose by glucose oxidase.