The Cell Meter™ Fluorimetric Phagocytosis Assay Kit (Deep red fluorescence) is designed to evaluate phagocytic activity in live cells. It is cellular process in which cells engulf and digest particles, such as pathogens or cellular debris, to maintain homeostasis. It features a ready-to-use suspension of Zymosan particles conjugated to Protonex™ Red 670, a novel pH-sensitive dye. Unlike traditional fluorophores, rotonex™ Red 670 is non-fluorescent at neutral pH, becoming highly fluorescent only upon acidification making it ideal for monitoring internalization and lysosomal fusion during phagocytosis. Once engulfed, the Zymosan conjugates are trafficked into acidic phagosomes/phagolysosomes, where the rotonex™ Red 670 dye is activated. The spectral property of the dye is similar to that of Cy5, allowing for seamless integration into existing Cy5-compatible detection workflows. The kit also contains a green fluorescent viability dye to enable multiplexed analysis of live cells and phagocytosis within the same assay. This two-color system provides a powerful and versatile tool for studying the dynamics and regulation of phagocytosis in various biological and immunological contexts. Suitable for applications in immunology, drug screening, and cellular function research, the assay is compatible with fluorescence microscopy, flow cytometry, and microplate-based formats. The kit also contains a green fluorescent viability dye to enable multiplexed analysis of live cells and phagocytosis within the same assay. This two-color system provides a powerful and versatile tool for studying the dynamics and regulation of phagocytosis in various biological and immunological contexts. Suitable for applications in immunology, drug screening, and cellular function research, the assay is compatible with fluorescence microscopy, flow cytometry, and microplate-based formats.