Lambs were injected parenterally with formalized cell extract, or bacterin, prepared from sporulating cultures of an enteropathogenic strain of Clostridium perfringens type A. This procedure induced an antibody in the serum which neutralized the erythemal and enteropathogenic activities of the cell extract. Immunodiffusion of the serum against the toxic cell extract produced a single, specific band associated with enteropathogenic activity. Protective immunity was not evident when the immunized lambs were challenged by the intraduodenal and the ligated intestinal loop methods with toxic cell extract or with whole cells of C. perfringens. Non-immunized lambs, challenged nine times by the intraduodenal method over a period of 70 days, had no measurable antibody and remained susceptible to experimental enteritis.