The light chain ratios and the concentrations of immunoglobulin G (IgG), IgA, and IgM were measured before, during, and after antileukemic therapy in 10 patients with common acute lymphoblastic leukemia. The concentrations of IgG, IgA, and IgM decreased substantially during treatment but recovered slowly after cessation of the therapy. The light chain ratios were not systematically affected, but at diagnosis the kappa/lambda ratios of total serum Igs, IgG, and in particular IgM were somewhat lower in the patient group compared with an age-matched reference group. It is concluded that, despite a decrease in serum Ig concentrations, virtually normal kappa/lambda ratios are preserved, indicating that kappa and lambda syntheses are affected to the same extent. These ratios remained normal for age during the recovery of the serum Ig concentrations; the features as described for the development of the light chain ratios in childhood were not observed.