The early detection of microbial contamination is an important issue in the production of biopharmaceuticals using animal cell culture systems. A new method, based on the ChemScan RDI analyser, was evaluated for the rapid detection and enumeration of low concentrations of microorganisms within a large population of animal cells and in cell culture media. The method was tested with suspension cell cultures and is applicable to adherent cell cultures. In both cases, the method uses an initial step to eliminate the animal cells followed by collection and fluorescent labelling of the viable microbial contaminants on a filter membrane. Total counts of the viable microorganisms were obtained after analysis of the membrane by the ChemScan RDI analyser. The results showed that the ChemScan RDI detected individual bacterial cells after filtration of the pre-treated animal cell culture. The detection limit of the ChemScan RDI was less than 10 bacteria/ml in cell culture containing 10(6) mammalian cells/ml and one bacterium in 500 ml of cell culture medium. A strong correlation between the standard plate count and the ChemScan RDI was observed, even at low bacterial concentrations. The total time for each analysis was less than two hours.