Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) is now under active investigation as a neuroprotective and anticancer agent. In the present study, the induction period method was used to investigate the antioxidant activity of ebselen in the radical polymerization of methyl methacrylate (MMA) at 70 degrees C. The reaction of ebselen with growing MMA radicals (lipid radicals) showed a k(inh) of 4 x 10(4) M(-1)s(-1). This value was similar to that for mercaptomethylimidazole (MMI, a thiol) and 10-fold greater than that for butylated hydroxyanisole (BHA). The ratio of the rate of chain inhibition to that of chain propagation (CI/CP) for ebselen, MMI and BHA was 0.1, 0.01 and 0.001, respectively, whereas the stoichiometric factor (n, the number of free radicals trapped by one mole of antioxidant moiety) for the corresponding compounds was 0.02, 0.2 and 2, respectively. Ebselen preferentially affected CP rather than CI, indicating that it was an effective scavenger (suppressor) of growing MMA radicals. These results suggest that ebselen is a potent suppressor of polyunsaturated fatty acid (PUFA) radicals, which are harmful radicals in biological systems.