Continuous cell lines obtained from mouse and hamster embryonic tissue and human lung contain a homologous series of acidic glycolipids known as gangliosides. These complex lipids are highly concentrated in plasma cell membranes. The ganglioside pattern becomes greatly simplified when cells are transformed by tumorigenic DNA and RNA viruses. Non-transforming viruses and lytic infection of cells by tumorigenic viruses without neoplastic tranformation do not cause changes in gangliosides. The alterations of ganglioside composition are the result of specific blocks in the biosynthesis of these lipids. The significance of these impairments of membrane glycolipid formation and the nature of the regulatory site(s) involved are considered with regard to the altered social behavior of transformed cells.