1. An original agglutinating potency for Proteus X-19 (Weil-Felix reaction) not exceeding dilutions of 1–40 was enhanced to a potency of 1–160 and, feebly, 1–320.

2. The horse's serum, after immunization, exerts distinct protective action against the Mexican virus, whether mixed with the virus before injection, administered 1 week or somewhat longer before the virus or given 24, 48 or 72 hours after infection with virus. Experiments of the last category have been most successful when the serum was given subcutaneously.

3. The serum of the immunized horse agglutinates the Weigl louse vaccines, containing Rickettsia prowazeki of the European disease in dilutions comparable in potency to the Weil-Felix reaction.

Since the serum described exerted protective effects when given 7 and 13 days before infection with the virus, there is some prospect of prophylactic usefulness on the part of this serum.

Since the serum also protected when subcutaneously administered 24, 48 and 72 hours after intraperitoneal infection of guinea pigs, and somewhat modified the disease even when given 96 hours after infection—that is 1 day before the control came down with a typical reaction—, it is at least logical to investigate its possible therapeutic value by early administration in the human disease of the New World or Mexican type. The time intervals governing the experiments of the last two categories in guinea pigs were of course dependent upon the balance between the amounts of virus and of serum used.

The failure of similar effects upon the European virus is not easily explained. This is especially difficult to understand in view of the unquestionable agglutinating potency of the serum for the Weigl vaccines. Whether this difficulty is purely a quantitative one, or depends upon other factors, can be determined only by further investigation.