An enzyme-linked immunosorbent assay is described for measuring kappa and lambda light chains within each of the serum immunoglobulin classes G, A and M. The detection limit was 0.06 U/L for total IgG, IgG kappa and IgG lambda, 0.2 U/L for total IgA, IgA kappa and IgA lambda and 0.5 U/L for total IgM, IgM kappa and IgM lambda. The concentrations of kappa plus lambda light chains from the three different immunoglobulins correlated well within those of total immunoglobulin G, A and M as measured by enzyme-linked immunosorbent assay or by immunonephelometry. Adult values for the kappa/lambda light chain ratio were found to be 2.0 for IgG kappa/lambda, 1.1 for IgA kappa/lambda and 1.7 for IgM kappa/lambda.