The newest technique in freeze-drying has struck an admirable balance between thermal contact for heat transfer and free surface for the escape of vapor. The limiting factor in classical freeze- drying is the input of heat without danger of melting. The application of heat from both sides, and the effect of expanded metal trays giving an area of contact of 20%, have produced a new limiting factor in the process--the removal of water vapor. Another limiting factor is the very poor thermal conductivity of the dry tissue.