The detection of bacterial infections in a mammalian cell culture process is realised using a gas sensor array. In production-scale and laboratory-scale cultivations of a perfused recombinant CHO-cell culture producing human blood coagulation Factor VIII, we show that the gas sensor array identifies bacterial contamination earlier than conventional methods. The sensitivity of the instrument is verified by inoculation of a blank cell culture medium with defined bacterial cell counts. © 2002 Elsevier Science B.V. All rights reserved.