The structure of the Rous sarcoma virus envelope glycoprotein complex was studied by sedimentation gradient centrifugation analyses of detergent-solubilized wild-type and mutant envelope (env) gene products. These studies show that the envelope glycoprotein forms an oligomer during biosynthesis, which is most likely a trimer, and that this is the form of the complex found in virions. Our results are consistent with oligomer formation and transport out of the endoplasmic reticulum being closely linked. From analyses of mutant envelope proteins we conclude that the extracellular domain of the glycoprotein is sufficient for oligomer formation but that the transmembrane domain is required to stabilize this complex. Additional experiments suggest that interactions between external domains of the membrane-spanning, gp37 polypeptides are those most important for the formation of trimers. The significance of these observations to retroviral replication and implications for antiviral drug development are discussed.