An extracellular glycoprotein (Pt60), produced in concentrations of about 10 mg l-1 in stationary cultures by Phoma tracheiphila, the causal agent of mal secco disease of citrus, was purified by affinity and ion exchange chromatography. Its molecular mass by SDS-PAGE was 60 kDa. Injections of 1-5 μg Pt60 into the mesophyll of sour orange or lemon leaves induced chlorosis and necrosis of tissues within 4-6 days. These symptoms closely resembled those caused by foliar inoculation with the pathogen. Results from carbohydrate analysis, following NaOH and N-glycosidase F treatments, indicated that Pt60 is a highly glycosylated protein (55% carbohydrate). Recognition by polyclonal antibodies, raised in rabbit against the whole glycoprotein, was strongly affected by the above treatments, indicating that the protein is enclosed by the antigenic glycosidic moiety. This carbohydrate sheath efficiently protects the protein moiety from degradation by proteolytic enzymes. The amino acid sequence of six peptides resulting from CNBr treatment of Pt60 followed by trypsin digestion did not reveal sequence homology with any known protein. Pt60 is thought to be the most toxic component of the malseccin complex which probably results from a mixture of several compounds, each differing from the others in the extent of glycosylation.