Comparative examination of a total of 1,030 blood samples from six turkey flocks of three Eastern Hungarian turkey farms was performed by the conventional haemagglutination inhibition (HI) and slide agglutination (SA) tests and by a competitive ELISA visualizing the inhibition by a positive test serum of the reaction between a monoclonal antibody and the specific epitope of Mycoplasma gallisepticum recognized by it. All the three tests detected the flocks which were certainly infected. The highest rate of positivity (93% of the samples tested) was revealed by the ELISA. By SA and HI the positivity rate was 56% and 55%, respectively. Thirty-five per cent of the positive blood samples reacted in all three tests, 17% of them only by ELISA and HI, another 17% only by ELISA and SA, while 3% only by SA and HI. In the case of positive flocks first the SA test and ELISA, then the HI test and ELISA give parallel results.