Specific for rat C-peptide only (Kd > 1 x 108 M). No cross-reactivity with human, bovine, porcine and mouse/rat insulin. Can be used for competitive ELISA/RIA detection of C-peptide in human serum after immunospecific removal of proinsulin; for construction of a sensitive two-site sandwich EIA/RIA immunosorbent assay for the measurement of total human proinsulin (including proinsulin convertion intermediates) in serum.