Under none of the experimental conditions here described was treatment of the infection induced by the virus of Western equine encephalomyelitis in mice and guinea pigs with specific hyperimmune rabbit serum effective if begun after the onset of signs of encephalitis. In mice, after intracerebral inoculation of virus, serum was ineffective when given even before that stage. After peripheral introduction of virus in guinea pigs the disease was completely arrested in certain animals by single or multiple doses of antiserum if treatment was begun within 24 to 48 hours after virus inoculation. In others the incubation period was prolonged to 2 or even as long as 7 weeks.
In untreated guinea pigs, injection of virus alone led to active immunity in those which survived. Antiserum blocked the antigenicity of active virus in the serum-treated animals. The decrease in titer in the sera of all antiserum-treated animals proceeded at the same rate as in those of control guinea pigs which received antiserum alone. Thus it was not possible to predict which ones would survive and which would succumb after a prolonged incubation period. Delayed fatal disease occurred at a time when the neutralizing antibody of the treated guinea pigs had fallen to a low titer. It is therefore likely that the virus which persisted throughout this long incubation period had been prevented from passing to and infecting other cells but reached them when antibody fell to an ineffective level in the surrounding medium. The relative frequency of such delayed reactions limits to a further extent the degree to which antiserum can be depended on for effective treatment of infection with the virus of equine encephalomyelitis.