Clostridium perfringens epsilon-toxin bound to the Madin Darby canine kidney (MDCK) cells and aggregated. The complex of the toxin was formed in a dose- and a time-dependent manner. The formation of the complex increased with a decrease in viable counts of MDCK cells and with increasing K+ release from the cells. The inactivated toxin heated at 100 degrees C did not aggregate under the condition. In addition, the prototoxin dose-dependently bound to the cells, but did not form the complex. Incubation of the toxin with MDCK cell membranes also showed the formation of the complex, but that with membrane preparations prepared from Vero cells or sheep erythrocytes, which are insensitive for the toxin, showed no formation of the complex. Incubation of the toxin with mouse brain homogenates resulted in formation of the complex, but that with brain homogenates heated at 80 degrees C or mouse liver homogenates showed no formation of the complex. These observations show that the complex formation of epsilon-toxin is essential for toxicity of the toxin.