SummaryThe use of the C-peptide radioimmunoassay has become possible due to the availability of synthetic antigen and improvement in antisera production.Proinsulin cross-reactivity can be avoided and C-peptide specifically determined by extraction of proinsulin with sepharose-bound insulin antibodies. In insulin treated diabetics, proinsulin insulin antibody complexes can be precipitated with polyethylene glycol (PEG) or concentrated ethanol. C-peptide immunoassay is leasable even in presence of exogenous insulin.With this assay it has been shown that1) almost half of the growth-onset diabetics have a residual insulin secretion, 2) residualβ-ceil activity is associated with a better diabetic control and 3) perhaps a lesser degree of degenerative complications.Finally, hypoglycemic conditions can be better documented by simultaneous determination of Cpeptide and insulin in blood.