Summary. The previous observation that Clostridium perfringens is responsible for penicillin‐relieved growth depression in chicks has been reassessed. The administration of Cl. perfringens to germfree chicks, to birds harbouring a non‐growth depressing gut flora and to birds reared in an environment free from Cl. perfringens was without effect on growth rate. No correlation was observed in conventional chicks between the incidence of growth depression and the presence of Cl. perfringens in the gut of the chicks one day after food was first given.