SummaryA competitive enzyme immunoassay has been used to detect and quantitate fibronectin in canine plasma. In this test, purified fibronectin, bound to microtiter plates, competes with plasma fibronectin for the conjugated antibody, rabbit‐anticanine, fibronec‐tin‐horseradish peroxidase. The assay could detect fibronectin in purified standards from 58 ng/ml to 580 μg/ml. The range of 1–100 μ g/ml was linear for plasma samples diluted 1:10, allowing samples with fibronectin concentrations from 10–1000 μg/ml to be easily measured by this method. The mean normal fibronectin concentration of 132 dogs, by this method, was determined to be 320