Clostridium perfringens type E iota toxin is implicated in some cases of fatal diarrhea in calves, lambs, and guinea pigs. A crossreacting "iota-like" toxin, produced by Clostridium spiroforme, is responsible for antibiotic-associated and weaning related enterotoxemias of rabbits. Antisera developed against culture supernatant of either organism neutralized the biological activity of iota or iota-like toxin. By using C. spiroforme antiserum and crossed immunoelectrophoresis (crossed IEP), we found two cross-reacting antigens in C. perfringens type E supernatants. C. perfringens types A, B, C, and D, which do not produce iota toxin, did not cross-react with C. spiroforme antiserum. To determine if either antigen had iota toxin activity, we separated the cross-reacting antigens of C. perfringens by preparative isoelectric focusing (IEF) and tested all IEF fractions for biological activity in guinea pigs and mice. The fraction containing the faster-migrating antigen seen in crossed IEP, designated iota b (i_b), had some guinea pig dermonecrotic and mouse lethal activity. Other fractions, including the one containing the slower migrating iota a (i_a) antigen, had little to no biological activity. When fractions containing i_a and i_b were mixed, there was an 8 and 25 fold increase in mouse lethal and dermonecrotic titers, respectively. Activity was neutralized by C. perfringens type E or C. spiroforme antisera and other fractions, when mixed with i_a or i_b, did not have a synergistic effect. Both components of C. perfringens iota toxin were purified using ammonium sulfate precipitation, DEAE anion exchange chromatography, preparative IEF, Sephadex G-100 gel filtration, and flatbed electrophoresis to yield a 12 and 5% final recovery of i_a and i_b, respectively. Each protein was homogeneous by SDS PAGE, gradient PAGE, and crossed IEP using homologous antiserum. There was at least an 8 fold increase in mouse lethal titer and 64 fold increase in dermonecrotic titer when equimolar amounts of i_a and i_b were mixed. Monospecific antisera against purified i_a and i_b neutralizd the iota or iota-like activity of crude supernatants. A sensitive and specific ELISA was developed using monospecific and C. spiroforme antisera. The i_a and i_b proteins have a pI of 5.2 and 4.2 and molecular weights of 48,000 and 71,000 (SDS PAGE), respectively. The i_a protein is heat stable (85° C/15 min) while i_b lost its activity at 55°C. Amino terminus sequencing revealed that both proteins were blocked by an unknown functional group(s). Purified i_a, but not i_b, has ADP-ribosylating activity specific poly-L-arginine in vitro. Recent evidence suggests that nonmuscle actin, involved in the cytoskeletal structure of eucaryotic cells, may act as the in situ acceptor. / Ph. D.