Serum antibody reactions were monitored for a period of 18 months in 18 heifers which had been vaccinated with Brucella abortus Killed 45/20 adjuvant vaccine, using four standard serological procedures: the Rose Bengal, Serum agglutination, Complement fixation and Coombs antibovine globulin tests. The Rose Bengal and Serum agglutination tests were persistently negative during the experiment. Titres to the complement fixation test rose sharply for only four weeks after the second dose of the vaccine. Thereafter the titres fluctuated to positive levels at unpredictable intervals. One out of 14 heifers tested 12 months after vaccination was strongly positive to the complement fixation test but all the animals were absolutely negative at the end of the experiment. The Coombs test became strongly positive after the second dose of the vaccine for up to 34 weeks after primary vaccination. The animals were negative to the test at the end of the experimental period but some still retained suspicious titres. It was concluded that the Brucella abortus killed 45/20 adjuvant vaccine did not interfer stall with the Rose Bengal and Serum agglutination tests, that the fluctuations of the Complement fixation titres to positive levels at unpredictable intervals were undesirable and that the Coombs test showed little promise as an assay which would be used to differentiate antibody titres due to Brucella abortus Killed 45/20 adjuvant vaccine from those of natural infection. It would appear that the Complement fixation test and the Coombs antibovine globulin test could be safely used 12 and 18 months respectively, after immunization of cattle with the vaccine.