A prospective study of lymphocyte subsets has been carried out for 18 months in 58 healthy first-degree relatives of Type 1 (insulin-dependent) probands. Subjects selected for presence or absence of islet cell antibodies included 10 with complement-fixing islet cell antibodies, 10 with conventional islet cell antibodies and 38 without islet cell antibodies. Immunoregulatory and effector lymphocytes subsets, and in particular activated T-cells, were investigated using a panel of monoclonal antibodies. The results showed no significant changes in total T, helper, suppressor/cytotoxic cell or K/NK cells. Activated T-cells were observed at least once in 22 subjects using the 4F2 monoclonal antibody and in 11 using the Tac antibody. Seven subjects had 4F2-positive cells on repeated occasions and one twice showed Tac-positive cells. Fluctuations and/or loss of islet cell antibodies were observed during follow-up. There was no correlation between presence of activated T-cells and either islet cell antibody status of HLA haplotype sharing with the diabetic proband. On the other hand, a significant correlation was observed between HLA-DR3 positivity of subjects and the occurrence of activated T-cells (both 4F2-positive and Tac-positive). We conclude that subjects with HLA-DR3 may be especially prone to T-cell activation. As none of the 'high risk' individuals developed diabetes in the course of follow-up, the relevance of these observations in the pathogenesis of Type 1 diabetes needs more prolonged investigation.