In this study, evaluation of three trials of vaccination of cattle against brucellosis through monitoring of the serological immune response and shedding of the vaccinal strains was carried out. For this purpose, a total of 84 Friesian cows were divided into three groups. The first group consisted of 42 cows that were vaccinated at the age of 5-8 months with Brucella abortus SRB51 vaccine and then revaccinated at the age of 20-22 months with the same vaccine. Animals of the second group (24 cows) were vaccinated at the age of 5-8 months with Brucella abortus S19 then revaccinated with RB51 vaccine at the age of 20-22 months, while animals of the third group (18) were vaccinated only at the age of 5-8 months with Brucella abortus S19. Sera of all vaccinates of the first group reacted positively at the second week post vaccination and some animals continued up to 19 weeks post vaccination using rough antigen. Employing smooth antigen, the conventional tests showed negative results. Sera of S19 vaccinated animals seroconverted at the 2nd week post vaccination and some continued up to 28 weeks post vaccination using the conventional tests. RB51 revaccinated animals in this group developed antibodies against the rough antigen up to 22 weeks post vaccination and no antibodies against the smooth antigen were detected. Examination of animals that were vaccinated only using S19 at 5-8 months after one year revealed that all animals were serologically negative. Bacteriologically, one cow from the first group shed the RB51vaccinal strain in milk three days post parturition and two cows shed the organism in their vaginal discharges. Cows that were vaccinated as calves with S19 and revaccinated as adults with RB51 showed no organisms in their milk or vaginal discharges