Fish (Sparus aurata) were intraperitoneally injected with model xenobiotics and several biomarkers of oxidative stress were analysed after 2 and 7 days exposure. The levels of soluble thiobarbituric acid reactive substances (TBARS) increased markedly in animals treated with polar xenobiotics, CuCl2 or paraquat; exposure to the apolar xenobiotics, dieldrin or malathion, enhanced significantly the microsomal TBARS while decreasing the microsomal glutathione transferase activity. The specific superoxide dismutase (SOD) activity increased in Cu(II)-injected animals but diminished in fish exposed to paraquat. After isoelectrofocusing separation and activity staining cell-free extracts of fish exposed to Cu(II), dieldrin or malathion displayed two new Cu,Zn-SOD isoforms of intermediate pI. An additional Mn-SOD was observed in dieldrin-injected fish, but only a faint new acidic isoform was observed in paraquat-injected animals. The new SOD bands were reproduced in vitro by incubation of cell-free extracts with systems generating superoxide anion or hydrogen peroxide and with a tert-butyl hydroperoxide/ADP-Fe system. Metallothionein induction was observed in Cu(II) or paraquat-exposed fish, but not in animals injected with apolar xenobiotics. So, the new SOD bands are possibly oxidized forms of this enzyme and can be considered as useful early biomarkers of oxidative stress due to transition metals or organic xenobiotics.