This work shows the presence of structural and physical analogies between some precise sites of the trimeric ectodomain of the transmembrane envelope glycoproteins of the HIV, SIV and FIV viruses and some precise sites of the interleukin 2 of the corresponding infected species (man, monkey, cat, respectively). In all the cases examined, the ectodomains of GP41 (HIV-SIV) and GP36 (FIV), because of their trimeric structure, place in the same spatial configuration the same amino acids clusters as the IL-2 amino acids clusters interacting with two, or even three, of the alpha, beta and gamma subunits of the IL-2 receptor. The molecular mimicry identified between the viral transmembrane glycoprotein and IL-2 is probably most useful for the virus and seems to be specific of each species; it is presumably due to a strategy common to the retroviruses associated with AIDS (HIV, SIV, FIV).