The green alga Chlorella sp. (strain 2350) was found to accumulate proline, up to a maximum of 0.027 fmol·cell Ϫ1 , under stressful concentrations of cupric ions. The function of the accumulated proline was studied with respect to its effect on copper (Cu) uptake. By induction with salt stress, cells with various levels of intracellular proline were used for this study. It is shown that the amount of Cu taken up by the cells was reduced when the intracellular proline levels were enhanced. When proline was exogenously supplied prior to Cu treatment, the adsorption of Cu was markedly reduced. When exogenous proline was supplied after Cu treatment, it resulted in a remarkable desorption of the adsorbed Cu immediately after the addition of proline. The results of the present study indicate that proline may exert some action on the cell surface and suggest that one function of accumulated proline is to reduce the uptake of metal ions. The accumulation of proline may be related to a tolerance mechanism for dealing with Cu stress.