Clostridium perfringens strain No. 28 and its penicillin-induced stable L form were treated with 10 different bacteriocins of C. perfringens. Viable count and labelled amino acid incorporation experiments revealed that the L form was sensitive to two and possibly three bacteriocins to which the bacillus was not, while both forms were commonly sensitive to two other bacteriocins and resistant to five others. Adsorption of bacteriocin, immunity factors, or perhaps uptake of bacteriocin might be proposed to explain the responses of these organisms to bacteriocins.