Armoracia rusticana is the commercial source of the enzyme Horseradish Peroxidase (HRP). Calcium ions play an important role in the functional conformation of HRP. The present study assesses the effect of three calcium salts viz., calcium chloride (CaCl₂), calcium nitrate [Ca(NO₃)₂] and calcium sulphate (CaSO₄) on the guaiacol activity of crude peroxidase extracted from the shoots and roots of in vitro grown plantlets of A. rusticana and their growth medium. The highest activity was observed in the shoot extracts of 25 mM CaCl₂ treated plantlets (1.92 U/mL) and the root extracts of 25 mM Ca(NO₃)₂ supplemented plantlets (2.84 U/mL). The crude peroxidase activity of the medium containing 25 mM CaCl₂ supplement was highest (0.13 U/mL). The capacity of the shoot and root extracts to decolourise a 10 ppm solution of methyl orange over 48 hours, was also tested. The decolourisation capacity was highest in the shoot extracts from CaCl₂ treated plantlets (49.32%) and root extracts from Ca(NO₃)₂ treated plantlets (29.72%) respectively. Hence, the addition of calcium salts to growth medium enhances both peroxidase activity and decolourisation capacity of crude extracts of A. rusticana plantlets. These findings are of significance in enzymatic treatment for decolourisation of effluents containing dyestuffs.