Values for the kappa/lambda-light chain ratio in immunoglobulins G, A and M and the total kappa/lambda-ratio, measured by enzyme linked immunosorbent assay, were evaluated in serum samples from different age groups (114 children, aged from 1 month to 15 years, and 20 adults). The IgG kappa/lambda-ratio decreased in the first 6 months and subsequently increased slowly during childhood towards the adult value of 2.0. The IgM kappa/lambda-ratio increased at a greater rate than IgG kappa/lambda-ratio in the first years of life and thereafter rose slightly throughout childhood to reach an adult value of 1.7. A decreasing IgA kappa/lambda-ratio was found from 1 month of age onwards to an adult value of 1.1. The pattern of total kappa/lambda-ratio was similar to the IgG kappa/lambda-ratio with an adult value of 2.0.