Intracellular toxin A and extracellular toxin A of Clostridium difficile were compared immunologically. Immunodiffusion tests with anti-intracellular toxin A and anti-extracellular toxin A sera showed that these toxins were identical. In neutralization tests, both antisera neutralized the homologous and heterologous toxins with regard to cytotoxicity, mouse lethality and loop response at nearly the same titers. Although intracellular toxin A lacks the hemagglutination (HA) activity, anti-intracellular toxin A serum neutralized HA activity of extracellular toxin A at the same titer as did anti-extracellular toxin A serum. These results suggest that these toxins are immunologically indistinguishable and that the intracellular toxin A molecule has an antigenic site(s) for the HA activity.