The preparation and characterization of intact native human proinsulin and C-peptide to serve as reference standards in immunologic and biologic assays is described. The major difficulty encountered in the preparation of human proinsulin from crystalline human insulin is the rather low yield (0.08 per cent), which may be due to postmortem autolysis in the pancreatic specimens. Under such circumstances, minor degrees of contamination of batches by fresh pancreases from other species, such as cows or pigs, may be magnified considerably. Such contamination can be readily detected with various currently available proinsulin- or C-peptide-specific antisera. Autolysis before or during preparation may also affect the yields and quality of human C-peptide preparations. Some of these C-peptide by-products are described, along with methods for their purification and characterization.

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