The activity of Bacillus brevis Nagano and the antibiotic it produces, gramicidin S, against the polyphagous plant pathogen, Botrytis cinerea was studied in vitro. Both germination and growth of B. cinerea were sensitive to the antibiotic although germination was the more sensitive. Sensitivity towards the antibiotic varied with cultural conditions. Treatment of B. cinerea conidia with high levels of gramicidin S caused massive release of cellular ATP indicating membrane disrupton as a primary mode of action. Activity of gramicidin S against conidia of B. cinerea was much lower when applied to leaves of Chinese cabbage compared to in vitro. This reduced activity was thought to be due to the binding of gramicidin S to ethanol soluble component(s) of the leaf surface. Gramicidin S also bound strongly to soil, especially the organic moiety. This gave the antibiotic low mobility and low bioavailability within soil. Methods were developed for the selective retrieval and enumeration of both B.brevis and B. cinerea from plant and soil material. Field trials showed that treatment of Chinese cabbage with B.brevis Nagano was as effective as standard fungicide treatment with iprodione in controlling grey mould infection in two successive field trials. This observation was not expected from the in planta studies concerned with antibiosis since it was indicated that gramicidin S had low activity on leaf surfaces. Observations of leaf wetness duration (LWD) indicated that B.brevis Nagano preparations contained a biosurfactant within the supernatant which altered the wettability of leaves. In greenhouse trials leaves dried 80 percent quicker after irrigation when treated with B. brevis Nagano culture compared to untreated control leaves.