The B 19 brucellosis strain has been widely used in the production of live vaccine against Bovine Brucellosis in the field for the last 30 years. Many attempts have been made to stabilize this vaccine but it remains a problem for laboratories which prepare this vaccine. In order to improve our knowledge of the influence of residual moisture and the nature of the conditioning atomosphere, several batches of vaccine were prepared in the same freeze-drying apparatus, either with a different atmosphere, or with a different level of residual moisture. After freeze-drying, the percentage of live bacteria and the stability of the vaccine at 37 degrees and 4 degrees C were investigated. It is thus possible to determine under specific conditions the level of residual moisture which gives: a) the best stability and b) the correct percentage of live bacteria after freeze-drying. Furthermore, by using nitrogen, argon, or a vacuum in the freeze-drying procedure, it was possible to select the most favourable atmosphere for conditioning the vaccine.