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Protoplasts isolated from an embryogenic callus line of `Femminello siracusano' lemon [Citrus limon (L.) Burm. f.] were incubated with 0.5 μm toxin of Phoma tracheiphila (Petri) Kanc. et Ghik., the pathogen of the mal secco disease, which seriously damages most commercial lemon cultivars. Two toxin-tolerant cell lines were obtained, and plants were regenerated from each line. The selected protoclones were tested for their tolerance by exposing callus and protoplasts to the toxin and detecting chitinase (a pathogenesis-related protein) among the intra- and extracellular proteins extracted from leaves of regenerated plants and suspension culture, respectively. The tolerance of the protoclones in these tests was equivalent to the tolerant lemon cultivar Monachello, and they were substantially more tolerant than their mother cultivar Femminello siracusano.