Of several strains of lactic acid bacteria which produced nisinase, Streptococcus thermophilus, strain TJ, was the most active. The optimum pH for extraction of the nisinase from an acetone preparation of bacterial cells and for incubation of the solution containing nisin and nisinase was in the neighbourhood of 7·0. The proportions of nisin destroyed by the nisinase under various conditions were determined. Also, it has been shown that nisinase has no action on a number of antibiotics, including subtilin.