Forty-four children with Type 1 (insulin-dependent) diabetes (aged 0.7-16.7 years) were observed from diagnosis for cytoplasmic islet cell antibodies and serum C-peptide concentrations. Islet cell antibodies were analysed by indirect immunofluorescence for both conventional IgG and complement-fixing antibodies. Thirty-seven children (84%) were found to be positive for conventional islet cell antibodies at diagnosis, and 21 (48%) remained positive over the observation period. Twenty-six patients (59%) were positive for complement-fixing antibodies at diagnosis and eight remained so during the follow-up period. The serum C-peptide concentrations increased significantly during the first 3 months after diagnosis, after which there was a gradual decrease in the levels. Those children who remained positive for complement-fixing antibodies over the observation period had significantly higher serum C-peptide concentrations on several occasions during the second year and had also a higher integrated serum C-peptide concentration over the initial 2 years than those who became negative for complement-fixing antibodies. These observations suggest that the continuous production of complement-fixing islet cell antibodies in those patients who are positive for these antibodies at diagnosis presupposes the preservation of a sufficient amount of functioning beta cells for antigenic stimulation. These results support the view that the complement-fixing islet cell antibodies reflect ongoing destructive processes in the beta cells.