To determine if sequence variations observed in cosmopolitan HTLV-I interfered with viral recognition by neutralizing antibodies, we evaluated the neutralization potential of sera from persons infected by HTLV-I of this clade selected for amino acid changes in their eny glycoproteins. Each serum was used to neutralize three previously described HTLV-I isolates, 2060, 2072, and 1010, that possess amino acid env sequences differing at several positions, one of them being located in the immunodominant and neutralizable domain (aa 187-199). The results obtained in syncytia and/or reporter gene inhibition assays showed that the neutralization pattern of the sera clearly differed and could be classified in three categories. Five sera completely neutralized the three viruses with an equivalent titer, two sera gave a maximum inhibition, with higher ID50 on the 2072 virus than on the 2060 or 1010 viruses, and three sera had a stronger neutralization potential toward the 1010 virus than toward the 2060 virus. One of these sera partially neutralized the virus produced by 2072 cells, whereas neutralizing antibodies in the other two recognized the neutralizable epitopes on the 1010 or 2072 viruses equally well. Identification of amino acid sequences involved in induction of neutralizing antibodies with different recognition capacities could help identify new neutralizable epitopes of HTLV-I envelope glycoproteins and to better define the component(s) of an effective vaccine.