Abstract
A survey of human sera from healthy individuals revealed the presence of naturally occurring antibodies that react in radioimmunoprecipitation assays with proteins of mammalian type-C viruses. Of 39 sera tested, 100% showed reactivity against baboon endogenous virus, whereas only 49% showed reactivity against simian sarcoma-associated virus. Polyacrylamide gel electrophoresis of immune precipitates revealed one to three bands that comigrate with the virus structural proteins. There were low, but detectable, levels of antibody to the major internal protein of murine leukemia virus, but no activity against the structural proteins of avian sarcoma virus.