IT has been thought unlikely that antibiotics are produced under natural conditions, (a) because the necessary nutrients would not usually be available and (b) because other micro-organisms might be expected to compete for nutrients and, in some cases, to destroy metabolically any antibiotic formed. Nevertheless, a plant parasite may have access to a relatively rich nutrient supply and may be, in early stages of an infection at least, isolated from the influence of other micro-organisms. Penicillium expansum is an unspecialized plant parasite, causing storage rots of apples; most freshly isolated strains produce the antibiotic patulin (synonyms : clavacin, expansine) in suitable culture media. We thought it of interest to see whether it produces patulin in apple tissues.