We studied the type specificity of the antisera which were produced in guinea pigs by immunization with heat-disrupted virions of human enteroviruses (echovirus types 7 and 11 and coxsackievirus type B5). When guinea pigs were immunized with heated virions (H particles), the antibodies produced reacted type specifically by complement fixation with the homologous H particles. By contrast, when guinea pigs were immunized with disrupted virions prepared by heating in the presence of sodium dodecyl sulfate, antibodies were produced which reacted broadly. Heterotypic complement-fixing antibody responses of humans to enterovirus infections are discussed.