A complex phenol-insoluble fraction (fraction P.I.) extracted from Brucella melitensis presents immunizing properties already described by the authors. This fraction contains peptidoglycane whose immuno-adjuvant properties are known in numerous bacterial species. More precise fractionation was carried out to determine if the properties observed after injection of P.I. are in fact specific or on the contrary due to peptidoglycane action. By chemical and enzymatic treatments, 90% of the P.I. are eliminated, leaving a 4A fraction having the same properties as P.I. and having the advantage of providing better protection. This protection specific to genus Brucella and slow in appearing, is not due to peptidoglycane. The 4A fraction consists essentially of peptidoglycane linked covalently with a lipoprotein and proteins of weak molecular weight. All attempts at more precise fractionation have until now led to the loss of biological activity.