Analysis of the records of instances in which non-immune persons contracted yellow fever notwithstanding vaccination shows that the onset of disease occurs soon after vaccination, the longest period being 13 days. Since the average incubation period in yellow fever is 6 days, it seems that infection must have taken place in some instances during the period while protection was developing. These instances led to a study of the possibility of immediate protection by means of the anti-icteroides serum. It had already been shown that the immune serum protects at once against experimental Leptospira icteroides infection, but it remained to determine how long the protection would last. Guinea pigs were given different quantities of the immune serum and subsequently injected, at various intervals, with a virulent strain of Leptospira icteroides. Complete protection enduring 5 days was obtained with as minute a quantity of serum as 0.002 cc. per 1,000 gm. of body weight. After 5 days, however, the immune substance rapidly diminished, and to keep the animal protected for as long as 10 days it was necessary to give 100 times as much, or 0.2 cc. For a man weighing 80 kilos, 0.16 cc. (0.002 x 80) would theoretically be sufficient to protect for at least 5 days, 1.6 cc. for 7 days, and 16 cc. for 10 days. This temporary protection may be a valuable antecedent to that furnished by vaccination, since the final effect of the latter cannot be expected until at least 9 to 10 days have passed.