Human C-peptide is determined by radioimmunoassay. On gel filtration of serum from a healthy subject and from a patient with islet cell carcinoma, C-peptide (MW 3025) appears ahead of insulin (MW 5808) and shows much higher molar concentrations than the hormone. Human proinsulin cross-reacts with our antiserum to synthetic human C-peptide. On direct determination of immunomeasurable C-peptide (IMCP) in fasting serum of 25 healthy subjects we find an average of 1.8 (+/- 0.4) ng/ml, corresponding to 60.4 X 10(-11) Mol/l. The molar concentration is about five-fold as compared to IMI (immuno-measurable insulin). IMCP and IMI patterns are not identical on stimulation of beta-cell secretion in healthy subjects by i.v. glucose or glucose-glibenclamide. This is probably due to differences in peripheral metabolism of both compounds. We conclude from our results that C-peptide determined in peripheral venous serum is a better indicator of beta-cell secretion than is insulin. Among 26 insulin-treated juvenile diabetics 15 show not measurable and 11 subnormal IMCP levels in fasting serum. No rise in IMCP is found 1-2 h following breakfast. Four juvenile patients receiving no insulin in a phase of total diabetes remission have normal or raised fasting IMCP concentrations. Only 2 out of 24 adult diabetics (16 treated with insulin and 8 with tablets) show non-measurable fasting IMCP concentrations, in another 4 patients values are below and in the remaining 18 cases above 1 ng/ml serum. Stimulation of beta-cell secretion through glucose-glibenclamide is more or less impaired in all adult diabetics compared to the healthy subjects.