There is an increasing evidence that autoimmune mechanisms may have a role in the pathogenesis in insulin-dependent diabetics. The numerical and functional study of peripheral blood lymphocytes in diabetes mellitus might indirectly contribute to the understanding of its pathogenesis. In this study, detection of peripheral blood T lymphocytes was measured by rosettes with sheep red blood cells (SRBC), and B lymphocytes were measured by immunofluorescence with specific antiserum to immunoglobulins. The mean (+/- SD) percentage of SRBC was 67.6 +/- 7.2 in 21 normal subjects, 71.5 +/- 7.0 in 15 insulin-dependent diabetics, and 68.6 +/- 6.7 in 30 insulin-independent diabetics. There was no difference in the absolute T-lymphocyte number per mm3 in these three groups. Insulin-dependent diabetics showed a normal percentage and absolute number of B lymphocytes when compared with normal subjects.