Metallothionein (rabbit liver)(MT),apo-MT, Zn₇-MT and Cd₇-MT were prepared respectively. The scavenging effects of Zn₇-MT and Cd₇-MT on hydroxyl radical were compared at various pH values, and at pH 6 the effects of Zn₇-MT were compared with related proteins and inorganic zinc salt. From these results it is concluded that the ability of Zn₇-MT to scavenge hydroxyl radical is far stronger than that of Cd₇MT and that the ability of MT to scavenge hydroxyl radical derives from reduced -SH group in MT.