Ascitic fluid fibronectin concentration was measured in 111 specimens by laser nephelometry. Sterile, portal hypertension-related fluid fibronectin concentration (24 +/- 14 micrograms/ml) was significantly lower than the concentration in infected, portal hypertension-related ascites (49 +/- 44 micrograms/ml, P less than 0.001), peritoneal carcinomatosis (123 +/- 45 micrograms/ml, P less than 0.001), massive liver metastases-related ascites (55 +/- 21 micrograms/ml, P less than 0.001), as well as in ascites of other types (94 +/- 42 micrograms/ml, P less than 0.001). The percentage of samples with fibronectin concentration, greater than 75 micrograms/ml, was 0% for sterile, portal hypertension-related ascites, 28% for infected, portal hypertension-related ascites, 89% for peritoneal carcinomatosis, 20% for massive liver metastases-related ascites, and 72% for ascites of other types. Ascitic fluid fibronectin concentration correlated in a linear fashion with ascitic fluid total protein (r = 0.81, P less than 0.001). Fibronectin concentration in ascites appears to be elevated under a variety of conditions and does not appear to be a specific marker for cancer.