SUMMARY:The action of various concentrations of nisin on the development of cells and spores from untreated and heated suspensions ofClostridium welchiiadded to gelatine has been investigated, using a tube colony count technique. The development of vegetative cells was prevented by the presence of 2 Reading Units (R.U.) of nisin/ml of the final culture medium, although the effective concentration may have been 6 R.U./ml. Approximately 40 R.U./ml prevented colony formation from spores. Gelatine containing nisin was dried. A considerable proportion of the activity of the antibiotic was still present after storage for 5 weeks.