Experiments using plant tissue cultures, such as measuring their tolerance to herbicides or metabolite analogs, elucidating biochemical pathways by exposing the tissues to metabolic inhibitors, or cryopreserving the cultures, require at some point the determination of the tissue’s viability. Viability is the capacity of a cell or organism to live, and the state of being viable is most commonly recognized by the growth of the studied tissues (1 -3 ). Measurements of growth can include measuring increases in fresh or dry weight of a cultured tissue (2 ,3 ) or measuring increases in cell number or size (2 –4 ).