Animal models and human studies have shown that conjugated dienes rise in the plasma after thermal injury. These dienes may serve as a marker of oxygen radical-mediated tissue injury. Twelve burn patients were randomized to receive the antioxidant enzyme polyethylene glycol-conjugated superoxide dismutase (PEG-SOD). Patients received either 500 or 1000 units per kilogram of PEG-SOD intravenously within 6 h of injury. Plasma samples were collected and conjugated diene levels were compared to diene levels of burn patients not treated and to diene levels from normal volunteers. Conjugated diene levels were increased in burn patients. PEG-SOD in either dose initially decreased conjugated diene levels in the plasma of both treatment groups. By 72 h, the diene levels increased in the 500 unit/kg group, but remained at near control levels in the 1000 unit/kg group for up to 200 h after injury. These data suggest that PEG-SOD is capable of preventing conjugated dienes formed as the result of oxygen radical production. It appears that 1000 units/kg is more effective than 500 units/kg in preventing conjugated diene formation.