The antioxidant responses of coffee (Coffea arabica L.) cell suspension cultures to cadmium (Cd) were investigated. Cd accumulated very rapidly in the cells and this accumulation was directly correlated with an increase in applied CdCl2 concentration in the external medium. At 0.05 mM CdCl2, growth was stimulated, but at 0.5 mM CdCl2, the growth rate was reduced. An alteration in activated oxygen metabolism was detected by visual analysis as well as by an increase in lipid peroxidation at the higher CdCl2 concentration. Catalase (CAT; EC 1.11.1.6), glutathione reductase (GR; EC 1.6.4.2) and superoxide dismutase (SOD; EC 1.15.1.1) activity increased, particularly at the higher concentration of CdCl2. Ascorbate peroxidase (APX; EC 1.11.1.11) activity was increased at the lower CdCl2 concentration used, but could not be detected in cells growing in the higher CdCl2 concentration after 24 h of growth, whilst guaiacol peroxidase (GOPX; EC 1.11.1.7) did not show a clear response to Cd treatment. An analysis by non-denaturing PAGE followed by staining for enzyme activity, revealed one CAT isoenzyme, nine SOD isoenzymes and four GR isoenzymes. The SOD isoenzymes were differently affected by CdCl2 treatment and one GR isoenzyme was shown to specifically respond to CdCl2. The results suggest that the higher concentrations of CdCl2 may lead to oxidative stress. The main response appears to be via the induction of SOD and CAT activities for the removal of reactive oxygen species (ROS), and by the induction of GR to ensure the availability of reduced glutathione for the synthesis of Cd-binding peptides, which may also be related to the inhibition of APX activity probably due to glutathione and ascorbate depletion.