C-peptide is secreted in equimolar concentrations with insulin, and is often measured to assess pancreatic β-cell function. C-peptide analysis is most often performed by radioimmunoassay (RIA) which has several disadvantages. We evaluated an automated, chemiluminescent immunoassay for C- peptide in terms of precision, linearity, interference, and correlation with a RIA method. The chemiluminescent assay demonstrated acceptable correlation with the RIA method (slope = 0.82, y-intercept = 0.88 ng/ml, r-value = 0.97). Between-run CVs ranged from 8 to 9%, which compared well with the RIA method. Linearity extended beyond the manufacturer's recommendations and recovery ranged from 87 to 112% across the concentrations tested, with a slope of 1.007. No significant interference was noted with hemoglobin, bilirubin, or triglyceride. Overall this method compared favorably with the RIA method and offers an alternative to RIA for the analysis of C-peptide. (C) 2000 Wiley- Liss, Inc.