Mechanical stress due to micro/ultrafiltration has been suggested by a number of authors as one of the sources for cell deactivation in continuous cell-recycle reactors. This work introduces cell deactivation in the modelling of cell-recycle fermenters. A two-population based model is used. It is shown that the kinetic parameters obtained from chemostat fermentations describe accurately the experimental results and the apparent deviations to the Luedeking-Piret relationship at low growth rates are explained. The model is applied to the data of two authors with dilution rates (D) ranging 0.2 and 1.05h(-1) and using either Lactobacillus rhamnosus or Lactococcus cremoris cells. Although only data for lactic fermentation were used, the proposed model is of general applicability and not restricted to a type of fermentation, a given apparatus configuration or Newtonian fluids. The development of improved procedures for biological reactor modelling, such as presented in this work, wall allow more adequate and efficient design and operation of these reactors.