A patient with chronic cold agglutinin disease and anti-I antibody was studied. The titer of the patient’s serum at 4°C was 700 with adult I RBC, 256 with cord RBC, 256 with adult i RBC. At room temperature the titers were decreased and the serum also reacted at a titer of 4,000 with enzyme treated adult I RBC. Dithiothreitol treatment of the serum and purified antibody decreased reactivity only slightly, indicating that the antibody was IgG. The heat eluate contained 1.2 mg/ml IgG kappa, but only 80 µg/ml IgM kappa. A hybridoma made by fusing the patient’s peripheral blood lymphocytes with a human myeloma cell line contained only IgM kappa (30 µg/ml) anti-I activity. The IgM fraction of the heat eluate reacted similarly to the hybridoma supernatant. The IgG fraction resembled the serum and heat eluate. In this case study, IgG kappa and IgM kappa immunoglobulins,both possessing cold agglutinin activity, were present in the patient’s serum. This finding is unusual in idiopathic cold agglutinin disease and in view of the predominance of an IgG cold agglutinin, splenectomy was recommended for treatment.