Although the control of immunity of cattle vaccinated against foot-and-mouth disease (FMD), performed by means of experimental infection, is a basic method, many laboratories have complemented or even partially replaced it by an indirect test (11, 15) consisting in detection of neutralizing antibodies in serum of immune animals, using kidney cell culture (14, 10, 16, 7, 5, 18, 23, 21) or mouse sucklings (9, 4). Owing to a distinct agreement found between immunity degree and antibody level (10, 21, 29, 7, 12, 13, 15, 23), in some cases, when it was not possible to make an infection test in investigating post-vaccination immunity, the seroneutralization method can prove irreplaceable. The aim of the present experiment was to verify immunity of grown-up cattle on the basis of neutralizing antibody level within 12 months after the last vaccination with trivalent FMD vaccine of A, C and О types.