The nature of the virus-neutralizing property which usually appears in the serum of animals recovering from an attack of poliomyelitis and which is found in the majority of convalescent human beings and so-called "normal" adult persons is problematic. Olitsky, Rhoads and Long (1929), and Schultz, Gebhardt and Bullock (1931) found that the serum-neutralization reaction was reversible. The latter workers suggested that the antibody is similar in some respects to the antitoxins and suggested that the neutralization reaction may be essentially of the toxinantitoxin order.