Recombinant therapeutic proteins are usually produced by cell culture technology using genetically modified host cell lines. During the manufacturing process, a mixture of the protein of interest and host cell derived impurities, including host cell proteins (HCPs) and other process related impurities are produced. Those process related impurities will be cleared or minimized though the process by optimization of process purification. Residual HCPs in the final drug substance may affect the quality, safety and efficacy and may result in clinical adverse effects. HCPs are typically quantified using immunoassays such as enzyme-linked immunosorbent assay (ELISA). The development and the validation of those assays are really challenging mainly due to the wide variety of possible HCPs in products. Although generic ELISA kits are commercially available to quantify HCPs from different recombinant systems, a process specific assay is required before drug registration and commercialization for biologics. The reagents, polyclonal antibodies and HCP standards, need to be carefully prepared and characterized to ensure a correct quantitation of the HCPs in the final product. Here we describe the first step of the development of a production platform-specific HCP-ELISA assay for UCB's biopharmaceuticals produced in a Chinese Hamster Ovary (CHO) cell line, i.e. the production of mock material containing the HCPs using a null cell line.