Trypan Blue Solution, 0.4%, is routinely used as a cell stain to assess cell viability using the dye exclusion test. This test is often performed while counting cells with the hemocytometer during routine subculturing, but can be performed any time cell viability needs to be determined quickly and accurately. The dye exclusion test is based upon the concept that viable cells do not take up impermeable dyes (like Trypan Blue), but dead cells are permeable and take up the dye.