Bleomycin in the presence of iron(II) degrades DNA to form a thiobarbituric acid-reactive product. This has been made the basis of a specific assay method for 'free' iron in biological fluids. Human synovial fluid, human cerebrospinal fluid and rat pleural-exudate fluid were found to contain micromolar concentrations of 'free' iron, which would be sufficient to allow formation of the hydroxyl radical from superoxide and hydrogen peroxide generated in vivo. This assay method does not detect iron bound to transport proteins or to enzymes.