To identify the antigenic determinants of Clostridium perfringens enterotoxin (CPE), overlapping peptides corresponding to the entire amino acid sequence of CPE were synthesized and their antigenicity and immunogenicity were analyzed. Of the 21 synthetic peptides (C-1 to C-21) tested, peptide C-7 (amino acid residues 91-110) showed the highest reactivity with rabbit antibodies against CPE. Peptides C-4 (46-65), C-9 (121-140), and C-11 (151-170) were reactive with the antibodies, whereas peptides C-2 (16-35), C-3 (31-50), and C-20 (286-305) were less reactive. Other peptides were much less reactive. On the other hand, all of the synthetic peptides reactive with the anti-CPE antibodies were found to elicit rabbit antibodies reactive with both their respective antigens and native CPE molecule. These findings demonstrate that CPE molecule contains at least 10 different antigenic determinants which are all immunogenic and supposed to be responsible for functionally and biologically active sites.