Sulforhodamine B is a sulfonated rhodamine derivative used as a dye in biological and medical research. It is widely employed in the sulforhodamine B assay (SRB assay), a colorimetric method for assessing cell viability, proliferation, and biomass, particularly in cytotoxicity testing and anticancer drug screening. In the SRB assay, it binds to basic amino acids in cellular proteins of viable adherent cells, and the bound dye is solubilized for quantification by absorbance at around 510 nm, which correlates with cell number. This makes it suitable for high-throughput screening. Additionally, due to its strong red fluorescence, photostability, and water solubility, it serves as a fluorescent tracer in environmental monitoring, fluid flow studies, and as a marker in microscopy and flow cytometry.