An important aspect in both process development and large-scale recombinant protein production from mammalian cells is the measurement of cell count and viability.  These parameters are used to monitor and compare cell culture performance.  Cell density and viability in CHO cell cultures are traditionally estimated using manual hemacytometer cell counts with trypan blue staining for viability.  These manual measurements are time consuming and variable.  The focus of the FDA’s PAT initiative is to improve process understanding and control through improved measurement and data analysis technology.  This presentation focuses on the evaluation of the Vi-CELL Cell Viability Analyzer (Beckman Coulter) for improving the cell count and viability measurements in mammalian cell culture.  The implementation and qualification strategy of the Vi-CELL for use in GMP manufacturing is also presented.  Using the Vi-CELL provides less subjective, more precise and more reproducible measurements as compared with traditional manual methods.  Because of improved precision and reproducibility, differences in cell culture performance can be more accurately assessed, improving process development decisions and ensuring process control in manufacturing.