Young (12 to 15 day old) mice are approximately as susceptible to the virus of equine encephalomyelitis, Eastern or Western strain, when it is given intraperitoneally as are adult mice when the virus is injected intracerebrally. With this susceptibility by the intraperitoneal route as a basis, the injection of immune serum-virus mixtures intraperitoneally was found to result in protection in dilutions which give rise to infection after intracerebral inoculation. The difference of protective power by the two indicated routes was shown not to depend on the amount of inoculum nor on the age of the intracerebrally injected mice. Incubation at 37 degrees C. for 2(1/2) hours neither increases nor diminishes the protective action of immune serum when the intraperitoneal method is employed. The phenomenon of selective protection in different tissues is elicited by the sera of hyperimmunized mice, guinea pigs, and rabbits and by sera derived from horses infected with the disease in nature or exposed to it by contact. Of four horses recovered from the malady, all showed antibody in their sera; of others exposed by contact, four of nine animals revealed antiviral bodies, when the intraperitoneal technique was employed. These tests on horse sera have pointed to the potential value of this procedure for epidemiological studies. Finally, the reaction itself has significance through its bearing on the mechanism of immunity.