As shown in the previous paper, the antitoxin content of the immune sera was titrated successfully in vitro by using partially purified toxin prepared from the toxin which was practically free from the interfering effect of the non-toxic heat-labile component. However, since it was sometimes difficult to obtain a good parent toxin, experiments were planned to produce a potent toxin and to measure various antigenic components (including toxin) in culture filtrates by flocculation. In these experiments, some media were proved to show less interference due to non-toxic components, especially the heat-labile one.