An activity which restores theta toxin activity in some theta toxin-deficient mutants of Clostridium perfringens.

Group a mutants of Clostridium perfringens are deficient in theta toxin but release a dialyzable substance ("substance A"), which restores theta toxin activity to group b mutants, into a culture supernatant; group b mutants are defective in "substance A" release. "Substance A" activity appeared in the exponentially growing phase of group a mutants and disappeared in the stationary phase. "Substance A" activity was most stable at pH 5.0 and 0 C and even increased threefold in the first 5 hr, but gradually decreased during the following 15 hr. It was quickly inactivated at neutral and higher pHs at 0 C.