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The need is well recognized for suitable reference populations for calibrating and verifying the size and concentration accuracy of particle analysis instruments for use in the measurement of suspended protein particles in biopharmaceuticals. Polystyrene bead standards are normally used as a reference material for calibrating and validating particle analyzers. However, these standards, unlike protein particles, are easily detected and do not challenge the sensitivity of optical instruments. Groups of instruments verified only with beads can still exhibit significant differences in measuring concentrations of more challenging protein particles. To minimize these and obtain consistent concentration measurements between instruments, reference populations must closely resemble protein aggregates in possessing high transparency and a refractive index close to typical protein matrix fluids. This paper describes work on evaluating a promising reference candidate and the use of this to harmonize the performance of Micro-Flow Imaging instruments. Results show that use of a suitable reference population can significantly increase measurement consistency when multiple instruments are used to characterize the same protein particle suspension.