Two commercial cultivars of Linum usitatissimum differed in their ability to form callus and then undergo shoot formation on inductive media. The linseed cultivar, Antares, produced a greater amount of callus at an earlier stage than the fibre cultivar, Belinka. After 28 days of incubation, explants of Antares formed large calli, of which over 80 % had developed a shoot-like structure. At the same stage, Belinka calli were smaller and only 30–40 % had developed shoot-like structures. Surface-associated peroxidases were extracted from calli at stages throughout their development by sequential vacuum infiltration/elution with water, followed by 100 mM CaCl2. This procedure gave samples of ‹secreted› and ‹ionically-bound› surface-associated peroxidases. These extracts exhibited activity against the peroxidase substrates o-dianisidine, tetramethylbenzidine and guaiacol. Total peroxidase activity of both cultivars was generally highest after 14 days when the explants were developing callus. After this point, the activity dropped to a minimum after 21 days and recovered to near initial levels between 28 and 35 days when shoot formation was in progress. These two stages of differentiation, stem explant to callus and callus to shoot, may involve the production of specific peroxidases. Using non-denaturing PAGE, seven cationic isozymes (C1-C7) and five anionic isozymes (A1-A5) were detected in extracts of Antares and Belinka. Of these, one cationic isozyme (C6) and one anionic isozyme (A1) were exclusively present in the early samples (14 and 21 days) during the dedifferentiation of explant to callus. Other isozymes were either absent in early samples (e.g. C5-Antares; C1-Belinka) or increased markedly in later stages (e.g. C7, C2 and C5 — Antares and Belinka) during differentiation of callus to shoots. Possible functions for these differentiation-specific peroxidases are discussed.