Fibronectin is a plasma glycoprotein implicated as an important opsonin in phagocytic host defense mechanisms involving the reticuloendothelial system. In critically ill patients and in response to starvation, plasma fibronectin levels are often reduced. This factor may be a significant determinant of morbidity and mortality in these patients. Intravenous lipid emulsions have been shown to accumulate in cells of the reticuloendothelial system after prolonged administration, and in some reports depress host immune functions. Our goal in this study was to determine if these changes were mediated through a reduction in plasma fibronectin levels. Plasma fibronectin was measured in 10 patients scheduled to receive intravenous lipid emulsion (10% Liposyn) as component of total parenteral nutrition. Liposyn was administered as a 10 hr infusion. Fibronectin levels were determined immediately prior to infusion and 1, 6, 12 and 24 hr later. In all patients, plasma fibronectin levels remained unchanged throughout the course of lipid infusion (pre-infusion=292±51; after 24 hr=298±21 μg/ml). In individual patients, levels varied less than 10% between time points. Our results indicate that reticuloendothelial blockade is not initiated by acute depletion of plasma fibronectin in patients receiving intravenous lipid emulsions.