It is difficult to understand why neutralizing antibodies against poliomyelitis virus are not always found in the serums of patients immediately after they recover from the acute stage of the disease. In a few instances in my experience they did not appear until a long time after recovery. There has been some parallel experimental evidence in animals, for it has been found that when a horse is inoculated with poliomyelitis virus some time may elapse before antibodies appear. With 2 horses it was found that the first specimens of blood, taken within a few weeks after the end of a series of injections of virus, not only failed to neutralize the virus but actually caused an accelerated production of the disease in Macaca mulatta monkeys.
The poliomyelitis antiserum obtained from 1 horse described previously1 contained potent antibodies which could neutralize not only homologous (Flexner's M. V.) but several heterologous