The invention relates to a process for detecting or determining a C-peptide-containing impurity in a sample of recombinantly produced human insulin or a derivative thereof, by a non-radioactive assay, comprising the steps: (a) preparing a sample of recombinantly produced human insulin or a derivative thereof; (b) mixing the samples with dilution buffer; (c) adding a tracer to mixture (b); (d) adding antibody specific for the C-peptide impurity to mixture (c); (e) adding "C-peptide second antibody bead" having at least one label to mixture (d); and (f) detecting or determining the presence of the C-peptide-containing impurity.