Ultrathin section and surface replica electron microscopy were applied in combination with immunoelectron microscopy to elucidate the fine structure of HIV. The shell of the tubular core shows p24 antigenicity, while p17 is located at the inner leaflet of the lipid membrane. The virus particle is studded with 70-80 protrusions. These knobs have a diameter of 15 nm, a height of 9 nm, and are probably arranged in a T = 7 I symmetry. The major envelope protein gp120 is spontaneously shed from the viral surface. A possible role of released gp120 in pathogenesis is discussed.