Sulforhodamine B (SRB) Cytotoxicity Assay is a sensitive, reproducible and easy-to-use assay based on the ability of SRB to bind to protein components of cells that have been fixed to tissue culture plates.
SRB is a bright-pink aminoxanthene dye with two sulfonic groups that bind to basic aminoacid residues under mild acidic conditions and dissociate under basic conditions.
As the binding of SRB is stoichiometric, the amount of dye extracted from stained cells is directly proportional to the cell mass. The fixed dye is solubilized and is measured photometrically at OD 540 nm with a reference filter of 690 nm. The OD values correlate with total protein content and therefore with cell number.