Residual beta cell function was estimated in 35 young diabetic patients (25 Blacks and 10 Indians) and 22 controls by means of C-peptide assays using glucagon as a provocative agent. The basal C-peptide levels (mean 0,32 +/- 0,23 nmol/l) and the 6-minute post-glucagon levels (mean 0,56 +/- 0,47 nmol/l) were significantly lower in the diabetic patients than in the controls (mean values 0,49 +/- 0,20 nmol/l and 1.24 +/- 0,48 nmol/l respectively). None the less, residual beta cell function, as gauged by the presence of significant C-peptide increments in response to glucagon provocation (mean increase 75% of basal level), was present in most of the patients. The Black patients had significantly lower basal and 6-minute post-glucagon C-peptide levels than the Indian patients.