The plasma C-peptide immunoreactivity (CPR) in 10 normal subjects varied considerably when measured with different antisera in parallel assays. The CPR level correlated with the blank “CPR” value measured in plasma devoid of C-peptide and to a lesser degree with the sensitivity of the standard curves obtained with the individual antisera.

Storage of plasma samples at different temperatures and for different lengths of time before the analyses were carried out resulted In further variation in the CPR results. This was caused by a time- and temperature-dependent fall in CPR, which was more pronounced with some antisera than with others. This sensitivity to storage of plasma did not correlate with the antigenk characteristics of the antisera as determined by their reactivity with 11 specific fragments of the C-peptide molecule.

The contribution of human proinsulin to the CPR concentration in normal subjects was considered to be negligible even though the relative immunoreactivity of human proinsulin and C-peptide ranged from 11 to 143 per cent among these antisera.

These results suggest that differences in C-peptide antisera are a major reason for the variation in the concentration of circulating CPR as measured in different, C-peptide immunoassays.

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