The 3′ untranslated region of the water buffalo Nramp1 (natural resistance-associated macrophage protein 1) gene contains two alleles (Nramp1A and Nramp1B), as detected by the denaturing gradient gel electrophoresis (DGGE) technique. The Nramp1BB genotype is associated with resistance of water buffalo to the intracellular pathogen Brucella abortus. This article provides evidence that the Nramp1AA genotype is associated with susceptibility to the same pathogen. Susceptibility or resistance of water buffalo to B. abortus was established by agglutination, complement fixation, and skin tests. The Nramp1 genotype was established by DGGE analysis. The association between the Nramp1AA genotype and susceptibility to B. abortus was demonstrated in two independent population samples (152 cases and 281 controls; 87 cases and 124 controls, respectively). Macrophages from Nramp1AA subjects displayed a lower Nramp1 mRNA level when compared with macrophages from Nramp1BB subjects. Also, monocytes and macrophages from Nramp1AA subjects displayed a higher number of viable intracellular bacteria in comparison with monocytes and macrophages from Nramp1BB animals, providing biological significance to the results from association studies.