Two methods were evaluated in order to assess the viability of protoplasts and cell suspensions of Coffea arabica
cv. Catimor used in a protocol of transformation by electroporation. One method consisted of staining with 1 % Evans blue and the other staining with 1% 3-[4,5-dimethyltiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT). When Evans blue was applied to viable cells and protoplasts they either did not stain or acquired a faint blue colour. However, it was difficult to distinguish the non-viable cells in the wide spectrum of clear blue tonalities. In contrast, with the MTT assay only the viable cells and protoplasts reduced this salt to the red coloured formazan; viable and non-viable cells were distinguished more clearly with MTT than with Evans blue. The optimal temperature for the reaction with MTT was 37°C . The time of incubation was shown to be important, since longer times improved the reaction; the highest viability value was obtained after incubation for 120 min.