The well-known viability assay with 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) is applied to plant cell suspensions. In this paper it is shown that parameters (pH, TTC concentration, incubation time) which are thought to be only dependent on different cell lines are underlying at least two additional functions (age of the culture and shear stress). Each cell in a different state of activity requires a new set of the parameters mentioned above. Furthermore the time-dependent formazan production courses vary to such an extent that they cannot be used for viability determination. Therefore the usage of TTC as a viability test implies non negligible errors compared to the Evans’ Blue staining method which does not involve cell metabolism. The values of the two different methods to determine the viability can differ by more than 50%. The data suggests abandoning the usage of TTC as a quantitative viability assay for plant cell suspensions.