Antigens encoded by the gag and env genes of the human T-lymphotropic virus type III/lymphadenopathy associated virus (HTLV-III/LAV) include a p55 gag polyprotein that yields p24 as the major virus core protein, and an env gene polyprotein, gp 160, that produces gp 120, the most immunogenic protein in humans, at the amino terminus. Although its use is limited to research laboratories due to the cost and specialized procedures involved, the analysis of sera by radioimmunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis is the test providing the optimal balance of specificity and sensitivity. Because the gp 120 represents the external virus protein, it would be the most appropriate antigen for vaccine development. Also viruses serologically related to HTLV-III/LAV were detected recently in two species of Old World monkeys. Because about half the healthy African green monkeys appear to have been exposed to simian T-lymphotropic virus type III (STLV-III), a related agent of the species, a characterization of the STLV-III gp 120 and immune response of the host may provide additional information for vaccine development.