Plasma concentrations of fibronectin, albumin, total protein and IgM were measured in 14 male patients undergoing aorto-coronary bypass operations. Fibronectin and IgM concentrations fell to 55% of the preoperative values 5 minutes after start of the extracorporeal circulation, and the same percentages were encountered 5 minutes after the termination of bypass. The concentrations had recovered to 75% of the preoperative values by the end of the operation. The plasma concentration of albumin was 68 +/- 9% of the preoperative value after 5 minutes of bypass, 66 +/- 7% at the end of bypass, and 83 +/- 9% of the end of operation (significantly different from fibronectin and IgM, p less than 0.05; Wilcoxon's test for paired differences). No correlation was found between the duration of extracorporeal circulation and the post-bypass concentrations of any protein (Kendall correlation). It is concluded that the fall of fibronectin concentrations during cardiopulmonary bypass can be sufficiently explained by dilution, and that a specific consumption of fibronectin does not occur. The less marked decreases of albumin and total protein were probably due to infusion of plasma-protein solution, which is poor in fibronectin and IgM.