Maleinimide was used as spin label for studying the effect of metallothionein (MT) on the conformation of erythrocyte membrane by ESR technique. The results show that the presence of different MTs resulted in considerable changes of the ESR parameters. W/S and τ_e. implying that MT can interact strongly with membrane. The experiments carried out in vitro demonstrated that MT could be absorbed on the surface of erythrocytes. In addition, rab-hits were in Jected s. c. with CdCl₂ to induce the biosynthesis of MT, the presence of MT mainly in the erythrocyte is first suggested after chromatographic separation of plasma and blood haemolysates from Cd-loaded rabbits. It is claimed that a dynamic equilibrium could be established between MT absorbed on the surface of erythrocytes and presence in plasma. The significance of the above findings is disscussed in brief.