Previous studies have indicated that human immunodeficiency virus (HIV) is enclosed with a lipid envelope similar in composition to cell plasma membranes and to other viruses. Further, the fluidity, as measured by spin resonance spectroscopy, is low and the viral envelope is among the most highly ordered membranes analyzed. However, the relationship between viral envelope lipids and those of the host cell is not known. Here we demonstrate that the phospholipids within the envelopes of HIV-1RF and HIV-2-L are similar to each other but significantly different from their respective host cell surface membranes. Further, we demonstrate that the cholesterol-tophospholipid molar ratio of the viral envelope is approximately 2.5 times that of the host cell surface membranes. Consistent with the elevated cholesterol-to-phospholipid molar ratio, the viral envelopes of HIV-1RF and HIV-2-L were shown to be 7.5% and 10.5% more ordered than the plasma membranes of their respective host cells. These data demonstrate that HIV-1 and HIV-2-L select specific lipid domains within the surface membrane of their host cells through which to emerge during viral maturation.Electron microscopic studies (1-3) have demonstrated that the human immunodeficiency virus (HIV) is surrounded by a lipid envelope derived from the host cell plasma membrane. The initial report of the lipid composition of the HIV demonstrated that the envelope phospholipids were similar to those of other enveloped viruses and the erythrocyte plasma membrane (4, 5). The demonstration of high levels of phospholipids normally found in cell surface membranes [sphingomyelin (Sph), phosphatidylethanolamine (PE), phosphatidylcholine (PC), and phosphatidylserine (PS)] supported the electron microscopic observations depicting the enclosure of the viral capsid with a lipid envelope derived from the host cell plasma membrane during the budding process (4, 5). Further, these studies (4, 5) and others (6) indicated that HIV had a low lipid-to-protein (weight) ratio and a high cholesterol-to-phospholipid molar ratio (C/P ratio).Electron spin resonance (ESR) analyses of HIV have indicated that the flexibility of an incorporated spin-labeled probe molecule (a 5-nitroxide derivative of stearic acid; 5-NS) is restricted and that the viral envelope is among the most "ordered" membranes analyzed (5,7,8). Order parameters, which were calculated to be >0.7 (when spectral recordings were made at 37°C), were comparable to those of other enveloped viral systems exhibiting high order parameters (>0.7) and C/P ratios >1.0 (5, 9-11).Although all HIV isolates examined to date exhibit high C/P ratios and elevated order parameters, differences in phospholipid classes have been reported. For example, in HIV isolates LK013, F7529, and 4105 (5) and HZ321 (9) Sph is 25-28 mol %, whereas in isolates HTLV-IIIB and HTLV-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 so...