Copper ions inhibit growth ofAmphora coffeaeformisbut are not lethal even after lengthy exposure at high concentrations. Growth inhibition is attributed primarily to copper bound to different types of sites on the cell surface which exhibit different binding strengths. Removal of weakly bound copper, by placing the cells in fresh medium or treating with EDTA, results in a partial reversal of toxicity. A minor fraction but important component of the external cellular copper is bound more tightly. When this copper is removed, by displacement with H+ions or cysteine, toxicity is completely reversed. Reversal of toxicity depends on the form of the copper moiety. Whereas copper toxicity may be readily reversed for copper (II) ions from either soluble copper salts or insoluble Cu2O and CuSCN it cannot be reversed for lipid‐soluble copper complexes. Entry of such complexes into the cell results in cell death.