The virus of equine encephalomyelitis may be recovered during the febrile period from both the cardiac and the peripheral blood (not washed erythrocytes) of experimentally infected animals, including the horse. It is rarely found after defervescence. It appears in the blood of the guinea-pig at about the 10th hr. after an intra-cranial inoculation of the virus, and at about the 33rd hour when the virus is given by the nasal route. It continues in the circulation until the decline in temp. and the subsequent prostration of the animal. The virus has been recovered from various parts of the central nervous system, and during the febrile period has been found in the liver, spleen, kidneys, adrenal glands and salivary glands of the guinea-pig. It has not been recovered from the filtered salivary secretions of guinea-pigs, from the concentrated filtrate of drinking water used by infected animals, or from the urine and feces of infected guinea-pigs. The disease can be transmitted by inoculation into the brain, the anterior chamber of the eye, the peritoneum or the veins, also when given intracutaneously or subcut. or by intranasal instillation. The results were negative after inoculation into rabbit testicles, into the conjunctiva and foot pads of the guinea-pig, after percutaneous injection and after feeding. By means of a standard neutralization technic it has been demonstrated that the 5 strains of the virus used in this study are identical. The same methods of in vivo and in vitro neutralization demonstrate antiviral substances in the serums of recovered rabbits and guinea-pigs, but not in that of the spontaneously infected horse that has recovered, unless after so-called hyperimmunization. Hyper-immune serum given intramuscularly will protect a guinea-pig against an intracerebral injection of the virus through the 4th hour, but not after the 10th, while it also is of therapeutic value when the virus is given by the nasal route. Such serum is of value as a prophylactic, since it protects guinea-pigs against intracerebral or intranasal inoculation if given from 24 hrs. to 3-4 days before the virus. A potent immune serum given intramuscularly in the amount of 0.5, 1, 2 or 3 cc, will protect a guinea-pig against a simultaneous injection of 0.3 cc. of virus suspension into the brain; 0.3 cc. of serum fails to protect. A definite correlation can be established between the appearance of the virus in the general circulation of the experimental animal and the time of most effective treatment with immune serum. Protection is unreliable after the virus has appeared in the blood.