The prerequisite of effective infection with the virus of human immunodeficiency (HIV) are the interaction between the CD4 complex of the susceptible cell and outer glycoproteins of the virus. The CD4 complex is found above all on the sub-population of helper inducer T lymphocytes and cells of the monocyto-macrophage line. These interactions are essential for the penetration of the virus into the cell and participate also in the fusion of infected and non-infected cells which is typical for HIV infection. The outer layer glycoproteins of HIV are immunogenic, however, the antibodies which are formed do not neutralize the virus. It is even probable that antibodies can under certain conditions facilitate the penetration of HIV into cells which have on their surface receptors for the Fc fragment of immunoglobulin, i.e. in the first place into cells of the monocyte-macrophage line. These cells play obviously a key role in HIV infection because they are resistant to the cytopathic action of the virus and during their numerous physiological functions serve above all as cells, presenting antigen to T lymphocytes and thus are repeatedly in close contact with other cellular elements of the immune system and may thus help to spread HIV in the organism.