Pancreatic beta cell function was measured in 15 nondiabetic controls, 10 insulin-dependent diabetics (IDD) and 19 non-insulin-dependent diabetics (NIDD), aged 18 to 45 years, by means of the peripheral serum C-peptide response to 1 mg of glucagon. The fasting serum C-peptide (FCP) in IDD was lower than in the controls and NIDD (p less than 0.01), but there was no significant difference between the controls and NIDD (p greater than 0.05). The maximal in crement of serum C-peptide (delta CP) after glucagon stimulation in the controls was higher than in IDD and NIDD (p less than 0.01), and there was a gap between IDD (less than or equal to 0.69 ng/ml) and NIDD (1.20 ng/ml). During the glucagon test, serum C-peptide concentrations were highest in the first 15 minutes unlike plasma glucose which reached its highest value between 20 and 40 minutes. NIDD, either obese or nonobese, had a lower mean delta CP value than did controls. In the controls, IDD and NIDD, the FCP was correlated well with delta CP (r = 0.61, 0.93 and 0.59) but not with fasting plasma glucose (r = 0.19, -0.08 and 0.23). During the glucagon test, the mean maximal increments of plasma glucose were between 52.5 and 62.5 mg/dl. Nausea was the main complaint in 19 (43%) of the subjects but it was mild and transient. In conclusion, measuring serum C-peptide response after glucagon stimulation is a simple and safe test which may be a discriminative method to establish insulin dependency in young diabetic patients.