The inactivating effect of methyl glyoxal on foot-and-mouth disease (FMD) virus was studied. The rate of inactivation depended upon the drug concentration, incubation temperature and pH of the medium. RNA recovered from the inactivated virus was not infectious. The complement-fixing activity of the virus was not reduced by methyl glyoxal treatment. The antigenicity of inactivated virus preparations determined by levels of virus neutralizing antibody in the blood sera of immunized white rats and rabbits was not inferior to that of the initial uninactivated virus.