The dye exclusion method is a type of cell viability test used to quantify viable cells present in a cell suspension. Trypan blue, Congo red, eosin, and erythrosine B are the most commonly used dyes for the dye exclusion method. These assays are based on the principle that live cells possess unbroken, intact cell membranes, which prevent the uptake of the dye, while dead cells lack this membrane integrity and allow the dye to enter the cell.