The isolated cell walls of the yeast Saccharomyces cerevisiae bound the heavy metals Cu2+, Cd2+, and Co2+ (0.40, 0.17, and 0.11 mu mol mg(-1) metal per wall dry mass). Chemical modification of the isolated cell walls modified their capacity to accumulate Cu2+ cations. The blocking of amino, carboxyl, or hydroxyl groups reduced the quantity of Cu2+ accumulated, indicating that they may each play a role in the binding of Cu2+. This in turn indicates that both the protein and the carbohydrate fractions of the cell walls are involved in heavy metal cation binding. Scatchard analysis of the binding of metal cations to both chemically modified and unmodified isolated cell walls indicates that there are different types of binding site.