Adjuvant for vaccines that comprises a non-lipidated bacterial outer-membrane polypeptide (Omp), in which the bacteria may be those of Brucella genus. The adjuvant may be a modified polypeptide or may, for example, be the Omp19S or the Omp16S polypeptide, parts thereof or mixtures of the two. In a preferred embodiment, the adjuvant is the non-lipidated polypeptide included in SEQ ID No: 1, or parts thereof. In a further preferred embodiment, the adjuvant is the non-lipidated polypeptide included in SEQ ID No: 2 or parts thereof.