Abstract: The invention relates to a polypeptide containing in its polypeptidic chain: the amino acid sequence of 101 amino acids of FIG. 8, or a fragment of this sequence, this fragment being such that it is liable to be recognized by antibodies also recognizing the abovesaid sequence of 101 amino acids, but it is not recognized by antibodies respectively raised against M. bovis, M. avium, M. phlei and M. tuberculosis, and possibly against M. leprae, M. intracellulare, M. scrofulaceum, M. fortuitum, M. gordonae and M. smegmatis; it is liable to generate antibodies which also recognize the abovesaid sequence of 101 amino acids but which do not recognize M. bovis, M. avium, M. Phlei and M. smegmatis; it reacts with the majority of sera from cattle suffering from Johne's disease; or the polypeptidic sequences resulting from the modification by substitution and/or by addition and/or by deletion of one or several amino acids in so far as this modification does not alter the above-mentioned properties.