Twenty dairy heifers each consumed 350 mg of chlortetracycline/day in their feed. Four tests were performed on serum specimens from these and 20 control calves after vaccination with Brucella abortus strain 19. The numbers of positive test results on the card test and mean titers on the tube and rivanol agglutination and complement-fixation tests were compared in the 2 groups. Using the rivanol and complement-fixation tests, there were differences in the mean titers at weeks 5 and 6 after vaccination, but by week 10, differences were not found. The results suggest that addition of low concentration of chlortetracycline in feeds have minimal effects on postvaccinal serologic reactions determined after strain-19 inoculation.