Process development in up- and downstream processing requires enhanced, non-time-consuming, and non-expensive monitoring techniques to track product purity, for example, the level of endotoxins, viral particles, and host cell proteins (HCPs). Currently, HCP amounts are measured by laborious and expensive HCP-enzyme-linked immunosorbent assay (ELISA) assays best suited for measuring HCP amounts in the low concentration regime. The measurement of higher HCP amounts using this method requires dilution steps, adding dilution errors to the measurement. In this work we evaluated the suitability of attenuated total reflection spectroscopy for HCP quantification in process development, using clarified cell culture fluid from monoclonal antibody producing Chinese hamster ovary-cells after treatment with different polyelectrolytes for semi-selective clarification. Forty undiluted samples were chosen for multivariate data analysis in the middle infrared range and predicted HCP-values were in good agreement with results obtained by an ELISA-assay, suggesting the suitability of this new method for HCP-quantification. As this method is able to quantify HCP titers ranging from approximately at least 20,000200,000?ng?mL-1, it is suitable especially for monitoring of process development steps with higher HCP concentrations, omitting dilution errors associated with ELISA assays. Biotechnol. Bioeng. 2013; 110: 252259. (C) 2012 Wiley Periodicals, Inc.