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 0C and even increased threefold in the first 5hr, but gradually decreased during the following 15hr. It was quickly inactivated at neutral and higher pHs at 0C.