Early studies were directed towards a better understanding of the early stages of AIDS retrovirus (RV) infection, such as adsorption as well as the later morphogenesis of new virus particles. These studies were carried out by the electron microscopic examination of the progressive changes produced by acute productive infection with the lymphadenopathy-associated-virus (LAV) strain. During the acute cytolytic infection, many crescent shaped budding particles as well as large numbers of mature particles containing variably shaped condensed nucleoid structures accumulated on the surface of the infected cells. We also examined the chronically infected 8E5 cell line which produces viral RNA and antigens but no infectious virus or reverse transcriptase (RT). Huge numbers of atypical particles were seen at the cell surface. These particles contained various amounts of nucleic acid but few particles containing the condensed nucleoid structures, so prominent during productive infection, could be found. Restriction enzyme analysis of an AIDS retrovirus isolated approximately 30 days after infection of an adult chimpanzee with a pool of five different AIDS isolates, clearly demonstrated the dominance of one of the viruses injected (the Ziare 2 strain) over the other four isolates. Using three different enzymes, no differences could be detected between a pair of isolates from husband and wife collected the same day. Similarly no gross differences between three isolates from a single AIDS patient could be detected.