The effect of hydrogen peroxide (H₂O₂) on catalase (CAT) isoform activities and amounts and on mRNA levels was studied in leaves from potato plants untreated and treated with homobrassinolide (HBR). Northern blot analysis revealed that 100 mm H₂O₂ supplied through the leaf petiole for 4 h did not induce CAT expression. In contrast, CAT1 and CAT2 responded differently to longer treatment, as CAT2 transcript levels increased markedly whereas CAT1 transcript levels remained unchanged. Western blot analysis showed disparity between the level of CAT1 transcript and CAT1 amount, which actually decreased after 28 h. CAT2 amount correlated well with transcript accumulation and CAT2 activity as visualised by zymogram analysis. H₂O₂ modified the relative importance of CAT isoforms. After 4 h, CAT1 was prevalent in untreated and H₂O₂-treated leaves. After 28 h, CAT2 was prevalent in H₂O₂-treated leaves; therefore, the quantified increase in total CAT activity in these leaves was due to the rise in CAT2. HBR pre-treatment increased CAT2 basal level not changing the pattern of CAT responses to H₂O₂, only lowering its amplitude. Even so, ultrastructural studies showed that HBR significantly reduced H₂O₂ negative effects on cellular sub-structures, allowing better recovery of affected structures and reducing the macroscopic injury symptoms on leaves, thus data point to a HBR protective role.