Healthy cells in culture contain maximal amounts of ATP and a wide range of metabolic stimuli do not increase these ATP levels. Any increase in the ATP concentration of a culture can therefore be associated with an increase in the number of viable cells. Counter to this observation is the finding that cytotoxic agents reduce the amount of ATP in a cell culture and any drop in the ATP concentration is always associated with a reduction in viable cell numbers. The key factor governing the ATP levels in mammalian cells is the size of the cell. LumiTech have performed extensive experiments which illustrate this, stimulating the metabolic activity of cells (for example neutrophils) has no effect on the detectable ATP concentration in a suspension of those cells.