Summary The effect of Fe nutrient availability on the isozyme pattern of the metalloenzyme system Superoxide dismutase (SOD) was studied in leaves from growing pea plants ( Pisum sativum L.). The total SOD activity of crude extracts only showed statistically significant variations at 40 days' growth in those plants cultured in low-Fe (8.9 nmoles · 1 -1 ) and excess-Fe (340.2 μmoles · 1 -1 ) solutions. But when the activity was expressed as percentage of the different SOD-isozymes (Mn-SOD, Cu/Zn-SOD I and Cu/Zn-SOD II), deficient and supraoptimum levels of Fe produced modifications in the balance between the leaf SOD isozymes depending on the plant age. At 20 days, those plants grown at Fe-excess levels showed an enhancement in both Mn-SOD and Cu,Zn-SOD I activities and a depression of isozyme Cu,Zn-SOD II, whereas in Fe-deficient plants no variations were found between the three isozyme proportions as compared with those of full nutrient plants. At 40 days' growth, the isozyme pattern of Fe-excess plants had not undergone significant changes, but under Fe-deficient concentrations Mn-SOD and Cu,Zn-SOD I activities were depressed and Cu,Zn-SOD II increased. Metal determinations showed that at 20 days' growth Mn concentrations were significantly enhanced both in plants under Fe-deficient and Fe-excess conditions, whereas at 40 days' growth a statistically significant decline in Mn was found in plants grown in those extreme Fe levels. The Cu concentration of leaves only showed variations in cultures with high levels of Fe. Foliar Zn increased in Fe-deficient media, but at 40 days's growth a depression in plants grown under supraoptimum Fe levels was found. These results suggest that the enhancement or depression of a determined SOD isozyme(s) is counterbalanced in the plant cell by changes in the activity of other isozyme(s). Such a balance, which could be exercised via the different interactions that take place between micronutrients, guarantees plant protection against the deleterious effects of Superoxide free radicals (McCord et al., 1971). The determination of SOD isozyme pattern appears to be useful for the differential diagnosis of Fe and Mn deficiencies.