In studies such as those on the duration of immunity following yellow fever vaccination, it has been customary to use the results of the mouse protection test as a measure of the immune state; and in order to procure a time series, sera taken at various periods are compared in the same protection test run. This immediately raises the question of the effect of the storage on the older sera in the production of any differences of behavior between old and recent material.
There has been no satisfactory answer to this question, although a tentative approach to the problem has been made from time to time. Lloyd and Penna (1) compared the antibody content of desiccated and fresh immune serum over a period of nine months and concluded that there had been no demonstrable decline in antibodies in the desiccated material. These comparisons were made between desiccated sera from rhesus monkeys and fresh sera from the corresponding monkeys at the later periods.