A prospective study was carried Out in order to determine the serological response to the official tests used in Chile for the detection of bovine brucellosis which are Rose Bengal (RB), Complement Fixation (CF) and Competing Elisa (C-ELISA), in adult cows vaccinated with two doses of vaccine strain RB-51 from a brucellosis free herd as certified by the Servicio Agricola y Ganadero (SAG) of the Ministry of Agriculture of Chile. A total of 100 female bovines older than 18 months were selected in order to be 90% confident that the proportion of positives in cows vaccinated twice would not exceed 2.95% if all collected serum samples remained negative. The animals were vaccinated subcutaneously with 2 cc of vaccine strain RB-51 in doses of 1-3.4x10(10)ufc with an interval of 30 days between the first and the second application; the date of the first vaccination was the day 0 of study. Serum samples were taken the days 0, 30 and 60. Samples were processed in the Laboratory of Reference for Bovine Brucellosis of the SAG, Osorno, X Region, Chile. None of the vaccinated cows showed serological reaction due to administration of two doses of RB-51 vaccine detected by routine tests for bovine brucellosis.