Data of the literature about frequency and specificity of giant cells during AIDS encephalitis are conflicting. These discrepancies could be explained by several factors. Envelope glycoprotein Gp 120, when free, induces the formation of giant cells through the fusion of cerebral monocytes. The concentration of this protein depends on the level of its production, function of the quantity and of the maturation of the HIV, and on the rapidity of its catabolism, due to anti-Gp 120 antibodies. On the other hand, the tropism of the virus for monocytes and lymphocytes could vary; this would lead to the formation of giant cells with a different life span. The fusion capability of HIV-2 might be lower on account of the smaller size of its Gp 120. The frequency of mutations in HIV lentivirus leading to the synchronous development of several different clones of HIV-1 in the same patient and the associated infectious diseases, due to risk factors common to the diseased population, current endemic or opportunistic agents, could explain the variability of the density of giant cells according to the country where the patients live.