Botrytis cinereahas been inhibited by pure cultures of many fungi and actinomycetes on lettuce extract agar and by these and by the mixed flora of soil suspensions on wounded detached lettuce leaves and on wounded leaves of living plants under conditions of high moisture and humidity. Control was exhibited in many cases at 4° C. though this was less marked than that at 15–25° C. Most fungal activity raised the pH to 7.8–8‐4 but the use of buffered media showed that most species were antagonistic because of the production of antibiotic substances independently of the high pH. Prior inoculation with soil suspensions was necessary to obtain control ofB. cinereaon wounded leaves of living plants although such suspensions and several fungi had given control on simultaneous inoculation withB. cinereaon wounded detached leaves.Saprophytic antagonism is responsible for some of the natural control ofB. cinereain the field. The amount of available moisture, which in turn is affected by the relative position of the dead tissue on the plant as well as by climatic factors, greatly influences the establishment and growth ofB. cinereaand the amount of microbiological antagonism likely to be exerted.Three‐weekly applications of suspensions ofCephalosporiumsp.,Fusarium flocciferumandPhoma eupyrenafailed to give a significant increase in the final survival of lettuce in the field. On the other hand, 3‐weekly applications of a dust containing tetrachloronitrobenzene gave a highly significant increase in survival, efficiency being greatly increased by the use of cloches.Earthing up induces early rotting of the petioles of ageing leaves and so probably prevents entry ofBotrytis cinereainto the stem. Increased survival in hollows as compared with ridges is apparently due largely to this earthing up effect and possibly also to increased microbiological activity on dead tissue under the wetter conditions.B. cinereacan enter the soil by invasion down the root systems of attacked plants and can attack other root systems in contact with these. Furthermore it has been recovered from the soil over 6 weeks after the disappearance of infected plants from above ground, despite the antagonism shown to be exerted by soil organisms.Control ofB. cinereaonEpilobiumsp.,Ranunculus repens, R. flammulaandLamium amplexicaulewas obtained by prior inoculation of killed tissue with soil