<p>Reliable particle counting is critical for applications such as clinical monitoring and cell therapy manufacturing. However, various particle counters using different measurement principles are employed, and the absence of reference materials makes standardization challenging. We conducted a round robin test to evaluate the accuracy and reproducibility of particle number concentration measurements using monodisperse polystyrene beads. The study material was prepared, characterized for homogeneity and stability, and assigned a reference value using the volumetric sample loop method. Internal comparisons showed that methods relying on counting beads and automated counters reported concentrations 9–10% higher than the reference value. The material was distributed to three instrument manufacturers, each employing automated counters. The reported results deviated from the reference value by approximately 10%. Feedback from participants highlighted calibration practices and flow cell artifacts as potential sources of bias. Expanded uncertainties, considering repeatability and method-specific factors, ranged from 8% to 11%, with all results falling within the expanded uncertainty of the reference value. This study demonstrates the importance of uncertainty estimation in interlaboratory studies and highlights the need for harmonized results in particle counting. The findings provide a basis for future standardization efforts in particle and cell counting methods.</p>