The present invention relates to neutralizing anti-HIV-1 antibodies, particularly to mAb 4E10-IgG1, which has an HIV-1 neutralizing potency comparable to the one of mAb 2F5 and 2G12. 4E10-IgG1 binds to a novel conserved epitope (NWFDIT) C-terminal of the ELDKWA epitope recognized by 2F5.1 appears that both epitopes are cryptic epitopes within a region that may be accessible in a virus-cell fusion intermediate state only. 4E10-IgG1 potently neutralizes tissue culture adapted strains but also primary isolates of different clades, including A, B, C, D, and E, inclusing viruses that were found to be resistant to 2F5. None of the tested isolates was resistant to both anti-gp41-antibodies. The invention therefore also relates to peptides containing the 4E10 epitope and to compositions made thereof, as well as to anti-idiotypic antibodies that are reactive with the paratope of 4E10-IgG1, to compositions containing an antiidiotypic antibody optionally in combination with a peptide containing the 4E10 epitope, and to anti-HIV-1 compositions comprising 4E10-IgG1, optionally in combination with another neutralizing antibody such as 2F5 and/or 2G12.