The polymeric forms of free light chains from the immunoglobulins have been estimated in serum from 10 healthy individuals and from 10 anephric patients. Light chains were also estimated in serum and urine from 29 patients with various degrees of renal insufficiency. The measurements were carried out by a radioimmunoassay. The mean concentrations of light chains in normal serum were found to be 4.9 mg/l for dimeric forms of kappa chains, 5.6 mg/l for monomeric forms of kappa chains, 5.1 mg/l for dimeric forms of lambda chains, and 2.7 mg/l for monomeric forms of lambda chains. The concentration of light chains in anephric man was increased to 5 times the normal level. The concentration of the different forms was 26.7 mg/l for dimeric forms of kappa chains, 29.6 mg/l for monomeric forms of kappa chains, 32.6 mg/l for dimeric forms of lambda chains, and 5.8 mg/l for monomeric forms of lambda chains. A minor amount of tetrameric forms of kappa chains was found. Gel filtration showed that a majority of the kappa chains in normal and anephric serum existed as monomers and non-covalently linked dimers, whereas the lambda chains mainly existed as stable, covalently linked dimers. In renal disease the serum concentration of light chains was found closely correlated to creatinine clearance. The 24-h urinary excretion of light chains was generally increased when the GFR was diminished regardless of the type of renal disease.