An indirect ELISA using a monoclonal anti-IgG1 enzyme conjugate for diagnosis of bovine brucellosis was evaluated. Three groups of sera from negative non-vaccinated, from negative vaccinated and from positive cows were used. The specificity of the ELISA for negative non-vaccinated cows was 99.2 and 99.6% for negative but vaccinated cows. The sensitivity of this technique estimated with positive sera was 99.5%. The ELISA evaluated appears to be a very useful tool in the diagnosis of bovine brucellosis, particularly for regions where little epidemiological information is available about this disease and where large numbers of sera should be tested to obtain such information.