An indirect enzyme immunoassay for detection of antibody to Brucella abortus in bovine milk was developed and validated using 6238 milk samples from Canadian herds (brucellosis free) and 202 samples from herds infected with B. abortus (from Argentina and Chile). The assay utilized lipopolysaccharide as the antigen, immobilized on the polystyrene matrix, whole milk to test and a mouse monoclonal antibody, specific for an epitope of bovine IgG1, conjugated with horseradish peroxidase. The sensitivity of the assay was 95.2% +/- 3.7% at a confidence limit of 95% for samples from B. abortus infected herds obtained from chile and 98.7% +/- 0.3% at a confidence limit of 95% for samples from similar herds in Argentina. Of the negative milk samples tested, 77 gave a result above the threshold value of 0.200 optical density units. When the 77 false positive samples were retested using 7.5 mM (final concentration) of EDTA and ethyleneglycol-bis-aminoether-N,N,N', N'-tetraacetic acid (EGTA), the number of false positive reactions was reduced to 3, giving a diagnostic specificity of 99.95%. The divalent cation chelating agents did not affect positive reactions and the sensitivity remained the same. Based on control samples included with each assay, the performance of the assay was consistent.