Trypan blue is the most common stain used in cell count and viability assays. Viable cells exclude the dye, while nonviable cells absorb the dye and appear blue. Cells should be in suspension as single cells in buffered saline before counting. Trypan blue can be used in assays providing total cell count, viable cell count or nonviable cell count alone. This solution is provided at the common concentration of 0.4% (w/v).