Fluid obtained from HeLa cell cultures infected with poliomyelitis viruses served as a complement-fixing antigen. When used in the native state, i.e. untreated in any way, the fluids acted as homotypically specific antigens. When heated, however, the antigenicity was broadened and a high degree of heterotypic reactivity was encountered.
Data are presented indicating that the observed group reactivity was apparently based on common antigens shared by the three immunologic types of poliomyelitis virus. This reactivity appeared to be specific for the poliomyelitis viruses. No evidence was obtained in preliminary experiments that heating of the antigens releases a "soluble" antigen responsible for the group reactivity.