The lymphocyte subpopulations of 26 insulin-dependent diabetics were studied. Thirteen of them had persistent pancreatic islet-cell serum antibodies (ICA) (mean (±SD) duration of diabetes 11 ± 8 years). The others were ICA-negative (mean duration of diabetes 10±8 years). The mean fasting blood glucose in the week before the lymphocyte count was 1.37±.45 g/1 (two specimens for every patient). As controls 19 healthy volunteers, sex and age matched, were investigated. The T-lymphocyte count was no different in diabetics compared to controls, B-cells were significantly raised (p <0.01) in the ICA-positive group, when tested with antihuman gammaglobulin sera (IgG + IgA + IgM), anti-human IgG and anti-human IgM, while no difference was observed between ICA-negative patients and normal subjects. IgA-bearing lymphocytes were equally raised in both diabetic groups (p <0.05). These data show an altered immunological balance in type IB (autoimmune) diabetes, characterized by an increased number of B-lymphocytes.