Cultures of Phoma tracheiphila produce an extracellular phytotoxic glycopeptide capable of inciting symptoms similar to those of mal secco disease in lemon shoots. The toxin was purified by Sepharose 6B column chromatography and affinity chromatography on a Concanavalin A-Sepharose 4B column. The estimated molecular weight of the toxin is 93 000 and it possesses an isoelectric point of 4·3. The carbohydrate portion (29·5%) consists of mannose, galactose and glucose. The peptide portion (36%) contains most of the common amino acids with aspartate, glutamate threonine and serine as major amino acids. Beta-elimination studies of the peptide followed by a reaction with sodium sulphite indicate that threonine and serine are involved in the sugar-peptide linkages. Anionic substances of the hydrolysed toxin reacted positively with carbazole reagent and suggest the presence of sugar acids in the toxin structure. The toxin is heat stable and is not host specific. Injection of pure toxin into lemon leaves caused necrosis and vein clearing followed by wilt and leaf shedding. Proteolytic enzymes such as pronase, papain or pancreatin abolished its biological activity. Production of the toxin increased during the stationary phase of growth.