Living or killed biological material such as bacteria, viruses, fowl-typhoid and B.C.G. vaccines is mixed with a drying medium comprising an aqueous solution of sodium carboxymethylcellulose and the resultant mixture is freeze-dried.  The drying medium preferably includes in the case of living bacteria &c. from 2-10% by weight based on the total volume of the drying medium, of an inert substance capable of retaining a residual amount of water, e.g. a sugar such as sucrose, and from 0.5-10% of a protective agent such as sodium glutamate (thought to act against carbonyl groups toxic to bacteria in a dried state).  The sodium carboxymethylcellulose should be one whose aqueous 1% w/v solution has a viscosity in the range 5-55 centipoises at 20  DEG C. and is employed in a concentration of from 0.5-2.0% by weight based on the total volume of the drying medium. The biological material is placed in vials or ampoules, is cooled to a temperature within the range -5 to -45  DEG C. and then freeze-dried to a moisture content below 1.2% w/w; secondary drying over phosphorous pentoxide may be additionally utilized.  The freeze-dried biological material may be sealed in oxygen-free nitrogen or under vacuum. Examples are directed to freeze-drying live cells of a strain designated 9R of Salmonella gallinarum suspended in a drying medium containing between 0.5% and 2.0% w/v of sodium carboxymethylcellulose, between 2% and 10% w/v of sucrose and between 0.5% and 10% w/v of sodium glutamate.  Between 20% and 35% of the live bacteria cells survived the freeze-drying process according to the various examples.ALSO:Living or killed biological material such as bacteria, viruses, fowl-typhoid and B.C.G. vaccines is mixed with a drying medium comprising an aqueous solution of sodium carboxymethylcellulose and the resultant mixture is freeze-dried. The drying medium preferably includes in the case of living bacteria &c. from 2 to 10% by weight based on the total volume of the drying medium, of an inert substance capable of retaining a residual amount of water e.g. a sugar such as sucrose, and from 0.5 to 10% of a protective agent such as sodium glutamate (thought to act against carbonyl groups toxic to bacteria in a dried state).  The sodium carboxymethylcellulose should be one whose aqueous 1% w/v solution has a viscosity in the range 5-55 centipoises at 20 DEG C., and is employed in a concentration of from 0.5 to 2.0% by weight based on the total volume of the drying medium.  The biological material is placed in vials or ampoules, is cooled to a temperature within the range -5 to -45 DEG C. and then freeze-dried to a moisture content below 1.2% w/w; secondary drying over phosphorous pentoxide may be additionally utilised.  The freeze-dried biological material may be sealed in oxygen-free nitrogen or under vacuum.  Examples are directed to freeze-drying live cells of a strain designated 9R of Salmonella gallinarum suspended in a drying medium containing between 0.5 and 2% w/v of sodium carboxymethylcellulose, between 2 and 10% w/v of sucrose and between 0.5 and 10 w/v of sodium glutamate. Between 20 and 35% of the live bacteria cells survived the freeze-drying process according to the various examples.