The growing need for foot-and-mouth disease (FMD) vaccine has caused a number of production laboratories to seek newer, more economical means of virus multiplication, which ensure in vitro cultivation. The oldest of these methods, developed by Frenkel in 1947-1951, is the method of multiplying FMD virus on the epithelium of cattle tongues (8). After improvement this technique was accepted by several institutes for industrial multiplication of FMD virus (15, 9, 23, 22). Multiplication of virus in kidney cell cultures in a monolayer or a culture suspension is a newer method. Kidney cells from calves (11, 12, 13, 18, 23, 24, 4, 1), piglets (21, 14, 12, 20, 17, 13), and cattle fetus kidneys (16) have been found suitable for this purpose. In recent years cultures of cell lines from BHK₂₁ (baby hamster kidneys) have been satisfactory for multiplying FMD virus in monolayers in statically held (bottles or in a rotary apparatus (22, 16, 25, 19), or in special reactors in which cells are multiplied in suspension (10, 3, 22). Many years of work and experiments proceeded the introduction of new methods into production on an industrial scale. The aim of our work was to investigate immunization values of experimental vaccine prepared from type C virus multiplied in cultures of calf kidney cells. Also, we wanted to determine the durability of the immunity on the basis of the level of neutralizing antibodies and experimental infection of immunized cattle.