We show the application of a novel optical on-line sensor fixed in spinner flasks for the online monitoring of dissolved O-2 concentrations during mammalian cell growth. Using this sensor that requires only minute changes to the flask to be made, we could determine the volumetric O-2 transfer coefficient as well as O-2 consumption rates. Under normal growth conditions the cells did not undergo O-2 limitation. Also, the transfer of O-2 from the atmosphere to the spinner flasks is influenced by the use of screw caps. The on-line measurement was further applied to determine the O-2 uptake rates which can then be used to monitor the metabolic state of the cells and also for online process monitoring.