The radial immunodiffusion test (RIT) was used in the study of the quantitative interrelations between three subtypes of type A of the foot-and-mouth disease virus--A5, A10, and A22. A reverse proportional correlation was demonstrated between the size of the precipitation circles and the antibody concentration in the homologous sera, and a direct proportional correlation between the diameter of the circles and the amount of the antigen used. It was shown that one and the same antigen produced precipitation circles of a different size with several series of homologous hyperimmune serum, depending on the content of antibodies in these sera. It was found in experiments with early sera obtained on the 7th day of a foot-and-mouth disease infection in guinea pigs as well as in investigations with antiserum 140 S of the foot-and-mouth disease virus A5 that the early and 140 S sera used in RIT had higher specificity as against the hyperimmune sera. Stated is the necessity of checking the specificity of the early sera, which often reveals antibodies of the 7 S class.