Fibronectin is a 440-kD MW glycoprotein involved in opsonization and adhesion of cells to collagen. We examined 13 episodes of peritonitis on 8 consecutive days from the start and once after recovery (control). Fibronectin clearance was compared to that of marker proteins for peritoneal transport. Increased dialysate fibronectin levels were observed on the first peritonitis day. They declined during recovery. Serum levels were stable during peritonitis. All protein clearances, including fibronectin, were increased during the acute phase and decreased during recovery. The clearance of fibronectin showed a time course similar to that of the marker proteins. It was in the range between IgG (150 kD) and alpha 2-macroglobulin (820 kD), thus as expected on the basis of its molecular weight. We conclude that elevated dialysate fibronectin levels observed during peritonitis are due to increased transperitoneal transport and not to local synthesis. This augmented transport results from increments in both effective surface area and intrinsic permeability of the peritoneum.