Fibronectin is an adhesive protein that can promote phagocytosis and endothelial cell adhesion. Plasma fibronectin declines following burn in animals and patients, potentially due to its complexing with circulating collagenous debris as well as its rapid binding to sites of tissue injury. Such depletion of fibronectin initiates an opsonic deficiency of the plasma. In view of the sensitivity of fibronectin to proteolytic enzymes, an additional factor that could contribute to the decrease of plasma opsonic activity after burn is the proteolytic fragmentation of fibronectin in the blood. In the current study, we determined if fibronectin fragments appear in the blood of anesthetized rats after a sublethal full-thickness skin burn of 15% to 16% of body surface. Plasma fibronectin concentration was quantified by enzyme-linked immunosorbent assay and the presence of fibronectin fragments in plasma was determined by immunoblot analysis. All blood was collected in an antiprotease mixture to yield final plasma concentrations of 0.15% EDTA, 3mmol/L phenylmethylsulfonyl fluoride, and 3 mmol/L iodoacetate to prevent degradation of fibronectin after sampling. Plasma fibronectin decreased 60% to 70% within 30 minutes post-burn, and this low level lasted for at least 4 hours. Within 30 minutes post-burn, two prominent fragments of fibronectin with a molecular weight of 110 +/- 2.2 kd and 122 +/- 3.3 Kd, respectively, were also detected in the plasma. Peak concentration of these fragments was detected at 60 minutes post-burn, but their level declined by 4 hours. By 4 hours, both bands appeared to resolve into doublets. To rule out the possibility that the fragments of fibronectin detected in the plasma were actually generated by coagulation enzymes activated at the site of peripheral blood sampling, rapid direct inferior vena cava sampling was performed, which also yield the presence of the fragments. Thus, fibronectin fragments exist in the plasma following thermal injury. Because fragments of fibronectin can compete with the intact fibronectin molecule with respect to its ability to stimulate macrophage phagocytosis, such fragments may contribute to altered systemic phagocytic host defense following thermal injury. Furthermore, because fibronectin peptides can compete with matrix fibronectin and impair adhesion of cultured endothelial cells, such circulating fragments may also influence the integrity of the vascular barrier.