Summary Antibody produced against preparations of VP1, one of the four structural polypeptides of foot-and-mouth disease virus, neutralized the virus and reacted with both full and empty particles in radioimmunoassays (RIA). Antiserum against VP2 reacted with artificial empty particles of the virus but not with full particles. In contrast, none of the individual polypeptides of poliovirus produced antisera which neutralized the virus nor reacted with it in RIA. However, antisera produced with VP1 and VP2 reacted with artificial empty particles in RIA.