This kit employs a solid-phase Enzyme-linked Immunosorbent Assay (ELISA) with a double-antibody sandwich technique to detect residual E.coli (Protein expression strains) host cell proteins (HCPs) in the sample. Polyclonal antibody specific to E.coli (Protein expression strains) HCPs was employed in the assay to capture any remaining HCPs in the sample. Both the Calibration Standard (or test sample) and the HRP (Horseradish Peroxidase) labeled with anti-E.coli (Protein expression strains) HCP antibody were simultaneously added to the pre-coated microtiter plate, and followed by incubation and washing steps. Then TMB (3,3',5,5'-tetramethylbenzidine) substrate added for reaction, HRP catalyzed the oxidation of TMB by H₂O₂ to produce a blue product (maximum absorption peak at 655nm). Afterwards the stop solution is added to terminate the enzymatic reaction, resulting in a yellow color product (maximum absorption peak at 450nm). The absorbance values at 450nm wavelength were positively correlated with the HCPs concentration in the Calibration Standard and the sample. The concentration of E.coli (Protein expression strains) in the sample can be calculated using the dose-response curve.