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 +/- 0.45 g/l (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 less than 0.01) in the ICA-positive group, when tested with anti-human gammaglobulin sera (IgG + IgA + IgM), 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 less than 0.05). These data show an altered immunological balance in type IB (autoimmune) diabetes, characterized by an increased number of B-lymphocytes.