AL721, which is a novel lipid mixture extracted from egg yolks, is believed to be a therapeutic pharmacologic agent. AL721 interacts with membranes of various types of cells with a common mode of action. AL721 modifies cellular membrane composition and fluidity through passive extraction and/or exchange of cholesterol. Physiologically diminished cell function due to rigidification of its membrane is reversible both in vitro and in vivo by AL721. Fluidization of aged membranes with AL721 has been shown to restore brain serotonin receptor function both in vitro and in vivo. AL721 can also successfully restore deficient immune responsiveness of lymphocytes to mitogen stimulation in aged subjects. Drug tolerance to morphine and ethanol develops upon elevation of the viscosity of neuronal cell membranes in order to counteract the fluidization effect of the drug. Treatment of rigidified cellular membranes with AL721 in vivo can markedly reduce withdrawal symptoms. The virucidal effect of AL721 on the human immunodeficiency virus is believed to operate by lowering of viral membrane cholesterol thus interfering with the binding of the viral antigen to the host cell. Non-toxicity of AL721 is clearly demonstrated in animal and human safety studies.