Freeze-drying or lyophilisation appears as an interesting alternative method for long time preservation of bacteria compared to cryopreservation in liquid nitrogen making it possible to sharply reduce the storage expenses. The freezedrying process involves three steps: freezing (water solidification), primary drying (ice sublimation), and secondary drying (moisture desorption) and these three steps could induce potential cellular damage. Improvement of the freeze-drying protocols of lactic acid bacteria (LAB) needs to identify and control the principal factors responsible of cellular injury. Our aim was to investigate and quantify the loss of biological activity of cell suspensions of Lactobacillus delbrueckii subsp. bulgaricus CFL1 following each step of the freeze-drying process. Concentrated suspensions of lactic acid bacteria were freeze-dried in a protective medium composed of 20% of sucrose. Some samples were removed after the freezing, the sublimation and at various times of the desorption steps. The residual moisture content, the acidification activity and the viability of the lactic acid bacteria were measured just after sampling and following various times of storage at 25 C of the dehydrated samples (7 and 15 days). The effects of the fermentation conditions (controlled pH and two media, MRS broth and wild whey based medium) and the process variables (chamber pressure, shelf temperature) applied during the sublimation step on the biological activity were also investigated. Sublimation appeared as the most cell damaging step of the freeze drying process and the losses of acidification activity were more pronounced when the bacteria were grown at controlled pH in MRS broth compared to the whey based medium. When considering the biological stability of the dehydrated samples, the influence of the residual moisture content and the process variables became more signifi- cant than the nature of the fermentation medium. Additional experiments are in progress to further investigate the cell injury mechanisms taking place during the sublimation step.