The metal affinity of Enterococcus hirae cell walls and the relation to the chemical composition of the wall constituents were investigated, by studying the nickel absorption on purified cell walls of Enterococcus hirae mutants. The strains of Enterococcus hirae are characterised by their penicillin resistance, the mother strains ATCC9790, the penicillin-sensitive mutant AS21 and the penicillin-resistant mutant R40 present an MIC (minimum inhibitory concentration of penicillin) of 1-2, 0.075 and 80 mu g cm(-3), respectively. Preliminary results showed that the penicillin-resistant cell wall (R40) had more affinity for nickel than the others. The maximum amount of fixed nickel average 2.03 mu mol of Ni mg(-1) of R40 cells walls, 1.06 mu mol of Ni mg(-1) of AS21 cell walls and 0.96 mu mol of Ni mg(-1) of ATCC cell walls. The Scatchard model showed at least two types of nickel fixation sites, with low and high affinity respectively, and a negative cooperativity between sites. Application of the BET isotherm implies a multilayer adsorption process in which one layer does not need to be completely saturated before the succeeding ones. The potentiometric titration curve showed the proton affinity distribution of the cell walls. Two protonation constants were obtained, one at pH 4.5 which corresponded to the pK(a) of a weak acid, and one at pH 7.2 which corresponded to the pK(a) of the couple of H2PO4-/HPO42-.