The general physical characteristics and replication of retroviruses are considered, along with assays for viral products. The specific agent for acquired immunodeficiency syndrome, the human immunodeficiency virus (HIV), is characterized as a lentivirus causing persistent, lifelong infection. While human immunodeficiency virus retroviruses share many of the same properties as other replication-competent viruses, genetic variability occurs among HIV isolates, and this variability may have a considerable effect on the virus' virulence, cell type specificity, viral susceptibility to antiviral compounds, clinical presentation, and disease progression. The most notable difference between HIV replication and other retroviruses is the intricate control of HIV gene expression by viral and cellular factors. Possible mechanisms by which HIV kills infected cells include the formulation of multinucleate syncytia; cytopathic components within the virions themselves; and interaction between viral envelope proteins and the CD4 molecule on the cell surface. Agents shown to inhibit viral replication at the level of the reverse transcriptase are phosphonoformate, sulfated polysaccharides, rifabutin, and nucleoside analogs, as well as purine and pyrimidine analogs. To date, only one nucleoside analog, zidovudine, has demonstrated clear clinical benefit and anti-HIV activity.