Two red macroalgae species, Gracilaria cornea and Chondrophycus poiteaui, were evaluated for their intra and extracellular Cd2+ accumulation capacity, photosynthetic response and thiol peptide production. Algae were exposed for 3 and 7 days to 0.1 and 1 microg CdCl2 ml(-1) (0.89 and 8.9 microM). Intracellular accumulation of Cd2+ by G. cornea was relatively low, only comprising 20% of total metal (intracellular+extracellular). In contrast, C. poiteaui accumulated intracellularly close to 100% of total Cd2+. In both species, metal uptake was dependent on the external Cd2+ concentration, metal exposure time and cell wall composition. In response to Cd2+ exposure, low amounts of thiol peptides were synthesized and the major difference between G. cornea and C. poiteaui was in the cell wall composition. The absence of insoluble polysaccharides in the cell wall of C. poiteaui suggested that this insoluble fraction might be involved in establishing an efficient barrier for the intracellular accumulation of Cd2+. This is the first study in which the cell wall composition, its influence on Cd2+ accumulation and intracellular responses in red macroalgae are evaluated.