THE discovery that the C-peptide of ox proinsulin is present in substantial amounts in the pancreas1 has led the way to the study of C-peptides from a variety of mammalian species. The isolation and elucidation of primary structure has been described for the C-peptides of pig2,3, man4-6, ox1,2,7,8, dog, sheep and monkey9, rat10-12, horse10 and duck13.