The present invention relates to a method for inducing antibodies neutralizing HIV primary isolates in a mammal, comprising administration of a pharmaceutical composition comprising a polypeptide represented by the formula N-L-C, in which: N represents the amino acid sequence 25 81 of gp41, C represents the amino acid sequence 112 157 of gp41, and L represents a flexible linking sequence comprising from 2 to 30 amino acids.