Clostridiunm perfringens was incubated in the presence of euflavine (EU); the resistant mutants which were thus isolated had highly reduced capacity to release a-toxin. This fact was confirmed by lecithinase determinations and by immunoelectrophoresis. Injected into guinea pigs and into & to 7-day-old chicks, these mutants were completely avirulent. A study of their properties indicated biochemical differences between wild types and mutants.