Previous studies suggest that Clostridium perfringens enterotoxin (CPE) inserts into mammalian membranes. Using Triton X-114 phase separation analysis and charge-shift electrophoresis, this study demonstrates that CPE exhibits the amphiphilicity required for membrane insertion, but this behavior develops only after exposure of CPE to membranes. This effect does not require proteolytic or covalent CPE modifications or formation of a previously reported 160-kDa CPE-containing complex. A novel 90-kDa CPE-containing complex with amphiphilic properties was detected in intestinal brush-border membranes and in CPE-sensitive, but not CPE-insensitive, cell lines using nondenaturing Triton X-100 electrophoresis. Immunoprecipitation analysis suggested that the 90-kDa complex is composed of CPE and a 45-50-kDa membrane protein. Since the 90-kDa complex is formed only in cells that bind and respond to CPE, these results are consistent with the 45-50-kDa protein mediating CPE amphiphilicity and serving as a functional CPE receptor. A four-step model for CPE action is proposed. 1) CPE binds to the 45-50-kDa protein to form a 90-kDa complex. 2) The 90-kDa complex undergoes some physical change corresponding to insertion or a conformational change. 3) The 90-kDa complex and a 70-kDa membrane protein interact to form a 160-kDa complex. 4) Formation of the 160-kDa complex leads to permeability alterations.