A four-year-old boy was diagnosed as having lymphoblastic leukemia in January 1967. On immunosuppressive therapy he had several clinical remissions and relapses. In mid-1968 he entered a period of sustained clinical remission. At that time a monoclonal gammaglobulin peak appeared in the electrophoresis pattern of his serum. The paraprotein which was present in concentrations above 44 mg. per ml., was characterized as γG with K-chain specificity. There was no evidence of multiple myeloma, and no urinary Bence-Jones protein was detected. The patient was investigated for cellular and humoral type immunologic responsiveness during the sustained remission period with normal results. The possible relationship between the paraprotein and the favorable clinical course in this patient is discussed.