We compared the ultrastructure of the human retroviruses by thin-section electron microscopy of infected lymphocytes. Virus particles form at the plasma membrane without involvement of a cytoplasmic precursor. Budding forms of human T-cell leukemia virus types I and II (HTLV-I and -II) consist of a crescent-shaped nucleoid separated from the envelope by an intermediate layer. Mature forms of these viruses are about 100 nm in diameter. The nucleoid is electron lucent and almost completely fills the virion. There is about a 10-nm space between the envelope and nucleoid. The envelope has fuzzy surface projections. HTLV-I and -II resemble other type C retroviruses in morphology. Budding forms of human immunodeficiency virus (HIV, LAV, HTLV-III) also have a crescent-shaped nucleoid but not an intermediate layer between the core and envelope. The envelope has rod-shaped surface projections. Mature forms of HIV have an electron-dense nucleoid that is eccentric and bar- or cone-shaped. Particles have the same ultrastructure as retroviruses of the Lentivirus genus. HIV is readily distinguishable from HTLV-I and -II by thin-section electron microscopy. HIV is usually found in extracellular spaces by transmission electron microscopy of thin sections, and scanning electron microscopy of HIV-infected T4 lymphocytes also shows many particles on the surface of these cells. Lymphadenopathy-associated virus type II (LAV-II) has the same internal ultrastructure as HIV, but its surface projections are more prominent, being about three times the length of those of HIV. Human T lymphotropic virus type IV (HTLV-IV) has the same morphology as LAV-II.