Fibronectin concentration was studied in plasma of mice at different times after inoculation of 3 different experimental tumors, Colon 26 adenocarcinoma, mFS6 fibrosarcoma and B16 melanoma. A significant increase in fibronectin concentrations 10-30 days after inoculation of all tumors was found. Highest and earliest fibronectin increases were observed in BALB/c mice inoculated with Colon 26 adenocarcinoma cells. Control animals inoculated with previously irradiated cells (5000 rad) showed neither tumor growth nor significant variations in plasma fibronectin levels.