This work aims at checking the possibility of estimating mammalian cell viability from images provided by an in situ microscope (ISM). It was found that images of cells in bioreactors obtained by a high-resolution ISM contain a certain part of cells which exhibits strong morphological similarity with images of dying cells or dead cells obtained by epifluorescence. Cell images of this fraction have less homogeneous texture and less smooth borders as compared to regular cells. Modifications of intracellular organelles and irregularities of the plasma membrane can explain such visual features. Therefore, by only using the texture effect, a criterion is proposed in order to distinguish living cells from the other ones. It is based on the variability of the inside part of the image of the cell. A quantitative estimate of viability (e(v)) is then calculated from a set of images obtained for each sample. The viabilities obtained from the conventional flow cytometry method are inside the 5% confidence interval of these estimations. (C) 2009 Elsevier Ltd. All rights reserved.