The clinical manifestations and immunologic features of a patient with plasma cell leukemia who produced k, IgG half-molecules are described. His serum contained both 7S myeloma protein and 4.3S half-molecules, whereas his urine contained predominantly half-molecules. The half- molecules were discovered because the serum and urine formed double precipitin lines when analyzed by commercially available IgG radial immunodiffusion plates that contained antibodies to determinants on both the Fab and Fc fragments. Immunoelectrophoresis also revealed double precipition lines with such antisera. In contrast, when antisera specific for the IgG Fc fragment were used, the serum showed only a single line formed by intact IgG, and the urine failed to react, indicating that the half-molecule was antigenically deficient in the Fc fragment. The half-molecule consisted of one covalently linked heavy and light chain, both having about normal molecular weights, suggesting that they did not have a large deletion which could have caused the half-molecule production. Comparison of the clinical manifestations of the patient with those of four other known patients who produced half- molecules suggested that half-molecule formation is not associated with a distinct clinical syndrome.