A compound with antibiotic activity toward Monilinia laxa was isolated from 10-day-old potato-dextrose stationary cultures of an isolate of Epicoccum nigrum, a component of the resident mycoflora of peach twigs previously found to antagonize Monilinia laxa. The ultraviolet, infrared, proton nuclear magnetic resonance and mass spectra of the purified antibiotic indicated that it was flavipin. Flavipin was active against a wide range of plant-pathogenic fungi and bacteria including M. laxa and several components of the mycoflora of peach twigs. The ED50 values for the germination of spores and the germ-tube growth of M. laxa were 17·52 μg ml−1 and 2·07 μg ml−1, respectively. Crude filtrates of M. laxa containing flavipin to a concentration of 200 μg ml−1 were stable in a range of pH 6–8 with temperature, O2 and light, retaining 30 % of activity against spore germination of M. laxa after 54 days at room temperature. The application of these crude filtrates to peach blossoms previously inoculated with M. laxa prevented blossom and twig infection, as did spraying with propagules of the fungus. © 1991, British Mycological Society. All rights reserved.