The immune response that develops subsequent to infection with human immunodeficiency virus (HIV) consists of both humoral and cellular elements that, when tested in vitro, can inhibit virus infection and syncytium formation and lyse virus-infected target cells. The antiviral immune response may thus represent one of the primary host-defense mechanisms responsible for the protracted asymptomatic phase of the disease that, in most patients, can last several years. An important question to be answered in the development of vaccine strategies against HIV is whether this response might be an effective barrier to de novo HIV infection.