According to Montagnier and his colleagues, definite proof that they have a retrovirus was obtained by isolating the particles and showing that in sucrose density gradients the particles band at the density of 1.16g/ml.

In fact Montagnier and many other researchers since then have claimed that the 1.16g/ml band contained "pure virus", that is particles isolated from everything else, as the title of his paper proclaims.

However, neither Montagnier nor anybody else, up till 1997 had published any electron micrographs to prove that the 1.16g/ml band contained purified virus particles or indeed any particles, viral or otherwise.