The invention relates to a polypeptide capable of reacting specifically with an anti-Toxoplasma gondii P30 protein antibody, and comprising a peptide sequence, in which any succession of 6 contiguous amino acids exhibits at most 67 % homology with the peptide sequence of said P30 protein, identified by SEQ ID No.1, or comprising a sequence derived from said peptide sequence, and the applications of this polypeptide especially for detecting a Toxoplasma gondii infection, in a biological sample.