An apparatus and method for dehydrating biological materials, such as vaccines and microorganism cultures, in which the materials are dehydrated in an evacuated container which is in a microwave waveguide that is open to the atmosphere. The apparatus comprises means for freezing the container of biological material, a microwave generator, a waveguide, means for introducing the container into the waveguide, means for applying a vacuum to the container and means for removing the dehydrated material from the waveguide. In the method of the invention, the container of biological material is put in a microwave waveguide open to the atmosphere, a vacuum is applied to the container, the material is frozen and is radiated to dehydrate it. The dehydrated material is then removed from the waveguide.