Abstract
Negative staining electron microscopy was used to study sucrose gradient-purified preparations of the simian immunodeficiency virus (SIVmac251). Both isolated and aggregated virus particles were observed together with some free-lying virus cores. The cores were 110 nm long and 25 to 50 nm wide and were mainly conical or wedge-like in shape. Surface projections were seen on the envelope membrane of many of the virus particles; the knobs were approximately 6 nm in length, 10 nm wide and from an end-on view they had a Y or triangular-shaped morphology.