A marked imbalance of the two light chain types of immunoglobulins was observed in two young male adults suffering from primary hypogammaglobulinemia and intrinsic factor-deficient pernicious anemia. In one patient, the kappa/lambda light chain ratio of serum immunoglobulins was 0.01; in the other, it was approximately 6 (normal value, 1.8 +/- 0.3). This light chain imbalance was found within each of the three main immunoglobulin classes. The number of mature immunoglobulin-producing cells in the bone marrow and in the mucosa of the gastrointestinal tract was reduced. The relative frequency of kappa- and lambda-producing cells in these tissues corresponded to the kappa/lambda ratio of serum immunoglobulins. However, the number of peripheral lymphocytes with membrane-associated immunoglobulins as well as the percentage distribution of blood lymphocytes with kappa- or lambda-type immunoglobulins on the membrane were within normal limits in both cases. The results suggested a hitherto unknown defect in the maturation of B-cells leading to an abnormal ratio of kappa- and lambda- type immunoglobulin-secreting cells.