Several lines of evidence underscore the important role of the humoral response specific for HTLV-I envelope protein in the protection against viral infection. One approach to producing efficient immunogens is to synthesize peptides corresponding to the primary amino-acid sequence of neutralizing epitopes found in the external sub-unit gp46. In this study, we have selected synthetic peptides overlapping the major linear neutralizing determinants described earlier and used them as immunogens in rabbits and mice. All rabbit polyclonal anti-sera raised against peptides recognized epitopes in a denaturated context as well as MAbs raised against the HB peptide (aa287-311). By contrast, synthetic peptides O (aa89-110), HH (aa190-209), T (aa190-212) and HB (aa287-311) have generated antibodies efficiently binding their epitopes in a native context, suggesting that these domains are well exposed both at the heterodimer and at the oligomer surface. None of the antibodies induced by synthetic peptides show in vitro neutralizing properties, even those with a good capacity to bind the native form of HTLV-I envelope proteins.