This E.coli (Protein Expression Strains) HCP ELISA assay Kit utilizes 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 samples. Polyclonal antibody specific to E.coli (Protein expression strains) HCPs was employed in the assay to capture any remaining HCPs in the samples. Both the Calibration Standard and test samples were simultaneously added to the microtiter plate coated with affinity purified capture antibody, and followed by incubation and washing. The biotinylated antibody was added to the microtiter plate to bind the HCPs, then reacted with streptavidin labeled with HRP (Horseradish Peroxidase). TMB (3,3',5,5' -tetramethylbenzidine) substrate was added into reaction, HRP catalyzed the oxidation of TMB by H₂O₂ to produce a blue colored product (maximum absorption peak at 655 nm). Then the stop solution was added to terminate the enzymatic reaction, resulting in a yellow colored product (maximum absorption peak at 450 nm). The absorbance values at 450 nm wavelength were positively correlated with the HCPs concentration in the calibration Standard and the samples. The concentration of HCPs in the sample can be calculated using the dose-response curve.