Spin-trapping agents have been used to study the involvement of free radicals in phenylhydrazine-induced haemolysis. Spin traps were found to decrease the rate of oxygen uptake and the rate of haemoglobin oxidation in the reaction of phenylhydrazine with oxyhaemoglobin. Spin traps were also found to inhibit haemolysis and lipid peroxidation in phenylhydrazine-treated fresh human erythrocytes, with a concomitant production of phenyl radical spin adducts. Lipophilic spin traps were found to be more effective inhibitors of haemolysis than their hydrophilic analogues.