A decreased concentration of immunoglobulin molecules of one light chain type (κ) was found in a young girl with recurrent respiratory infections and diarrhea. She had, in addition, decreased γA and γE globulin, partial albinism, and intestinal lactase deficiency. No eosinophils were detected in blood, bone marrow, or nasal smears. Radiolabeled κ- and λ -type molecules survived equally well, suggesting that the synthesis of molecules bearing κ-chains was decreased. The patient improved considerably with the regular administration of γ-globulin. It is possible that biological as well as chemical differences exist between immunoglobulins of the two light-chain types and that an optimal immune response is dependent on a normal complement of κ- and λ-chains.