Plasma fibronectin is an important opsonic protein of the mononuclear phagocyte system (MPS). In this study, plasma fibronectin concentrations were measured in stored and fresh whole blood, fresh frozen plasma and AGH cryoprecipitate. Stored whole blood had the lowest concentrations (151 +/- 50 mg/l). The concentrations in fresh whole blood plasma (227 +/- 66 mg/l) and fresh frozen plasma (224 +/- 78 mg/l) were similar but significantly higher than in stored whole blood (p less than 0.05). In contrast, AHG cryoprecipitate had the highest fibronectin concentrations (3249 +/- 502 mg/l) (p less than 0.001). In correcting fibronectin deficiency the best results can thus be expected from an infusion of cryoprecipitate, but fresh whole blood and fresh frozen plasma may also increase depressed fibronectin levels.