This report describes a unit employed for the freeze-drying of histocompatibility typing sera using a 50-h cycle. The unit will process approximately 3,200 3-ml vials with a final residual moisture content of less than 2%. The system employs dry ice-alcohol cooled circulating baths to maintain the condensers below -60 degrees C, and two shelf-cooling baths to maintain the product at the required temperatures during the freeze-drying process. The results of a recently conducted 5-year study of the effect of residual moisture as a function of time and storage temperature are also included. Studies conducted to date indicate that with residual moistures below 2%, freeze-dried histocompatibility sera can be stored at +4 degrees C without the loss of significant tissue typing factors. Solubility of all serum was lost when samples were stored at +37 degrees C or higher during this same 5-year period. The long-term storage studies are being continued.