We studied the effect of oxygen-free radicals on fracture healing. 30 male rats were divided into 2 groups: 15 rats were given saline 5 mL/kg i.p. (control group) and 15 were given zymosan 100 mg/kg i.p. to induce oxygen-free radicals through stimulation of NADPH oxidase in polymorphonuclear leucocytes. 1 hour later, the right forelimbs of the rats were broken by light manual compression. These treatments were given once a day until the fifth post-fracture day. All rats were killed on day 22, and histological sections of the radius and ulna were examined without knowledge of the treatment given. The administration of zymosan impaired the fracture healing and therefore oxygen-free radicals appear to play an important role in fracture healing.