In a study of 27 thermally burned patients (mean TBSA, 58%; range, 32–96%) serum fibronectin levels were decreased with parallel decreased oxygen consumption of stimulated peripheral blood phagocytes and decreased EGTA-blocked burn serum opsonizing activity which correlated with serum fibronectin changes postburn. Normal and burn sera fibronectin content also correlated with the opsonizing times for zymosan andStaphylococcus aureusbut not forEnterobacteriaceae.Although in vivo 14 cases showed circulating fibronectin 140 μg/ml or lower and a marked decrease inStaphylococcus aureusopsonization, only two patients from this group revealed positiveStaphylococcus aureusblood cultures and serum fibronectin levels were higher in patients withStaphylococcus aureussepsis than in patients withEnterobacteriaceaesepsis. Supplementary experiments on leukocyte oxidative response after zymosan stimulation in normal, fibronectin-depleted, and fibronectin-reconstituted serum demonstrated that the lag period of oxygen burst is a fibronectin-dependent reaction.