The Streptomyces carcinomycicus strain was able to reduce the incidence of Brassica campestris Linn. leaf spot disease to a considerable extent, when the culture filtrate or spore suspension of the antagonist was sprayed on leaf spot disease. The reduction in leaf spot disease was achieved by spraying the spore suspension of Streptomyces carcinomycicus on Brassica campestris Linn. plant at the time of inoculation with Alternaria brassicae (Berk.). The corresponding reduction when the culture filtrate instead of spore suspension was sprayed.