Plasma fibronectin was determined by laser nephelometric immunoassay in two populations: healthy individuals and patients with metastatic or non-metastatic breast cancer. The results showed that the fibronectin concentration was higher in the patient group than in the healthy controls of similar age, with a significant difference (p less than 0.05). The patients who had metastatic breast cancer tended to show higher levels than those with no detectable metastasis, but such a difference was not statistically significant. Since fibronectin is sensitive to clinical events unrelated to the malignancy status, it does not seem suitable as a tumor marker.