Certain cases of chronic lymphocytic leukemia are associated with monoclonal immunoglobulins in the serum. In the present study it was possible to demonstrate that the surface immunoglobulin of the leukemic lymphocytes was idiotypically identical to the serum monoclonal immunoglobulin of the same individual. This was done through the use of fluorescent antibodies that were prepared against the isolated immunoglobulin M kappa serum protein from patient Ei. Such antibodies, after absorption, failed to react with the lymphocytes of other individuals. Capping experiments indicated that both the immunoglobulin M and the immunoglobulin D on the leukemic cells possessed the same idiotypic specificity.

A population of plasma cells was also identified in the peripheral blood of patient Ei that showed strong intracellular staining with the idiotypic antiserum and was the apparent source of the specific serum immunoglobulin. The accumulated evidence indicates that the plasma cells were derived from the leukemic lymphocytes by a process of differentiation. Surface immunoglobulin D disappeared and surface immunoglobulin M decreased markedly in this process.