Plasma fibronectin is important for host defences as an opsonin of the mononuclear phagocyte system (MPS). The effects of three types of plasma substitutes, crystalloid, gelatin and dextran solutions, on plasma fibronectin levels were studied in patients undergoing abdominal surgery in relation to serum total protein and serum C-reactive protein (CRP) levels. Plasma fibronectin levels decreased during induction of anaesthesia and infusion of an electrolyte solution before infusion of the plasma substitute, and they were further decreased during operation after infusion of plasma substitutes and remained depressed on the first postoperative day in all study groups (P less than 0.05-0.01). In the crystalloid and dextran groups, plasma fibronectin levels exceeded preoperative levels by the 4th-5th postoperative day (P less than 0.05-0.01), but in the gelatin solution group they remained depressed (P less than 0.01). Serum total protein levels remained lower than preoperative values throughout the study period (P less than 0.05-0.01), whereas CRP levels were many times higher on days 1 and 4-5 postoperatively in all groups (P less than 0.01).