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 37ºC. 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.