The extent and the clinical significance of residual beta cell function has been evaluated by radioimmunoassay of C-peptide in 41 diabetic children in different stages of evolution, using an arginine tolerance test. In control subjects a significant rise of C-peptide levels occurred after the infusion with arginine. In patients at the onset of the disease and in patients not in the remission stage, C-peptide levels showed no increment and basal values were significantly lower than in healthy control children. Children during the remission phase showed basal and peak values not significantly different from controls. A positive correlation was found between highest CPR levels compared to basal CPR values and to the age at onset of diabetes; a negative correlation was found between the duration of the disease and insulin requirement.