The radical-scavenging activity of estrogens (estrone, 2-hydroxyestradiol), estrogen-like compounds (diethylstilbestrol, DES; bisphenol A, BPA) and the monophenolic compound 2,6-di-t-butyl-4-methoxyphenol (BMP) was investigated using the method of measuring the induction period for polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) at 70 degrees C using differential scanning calorimetry (DSC). The stoichiometric factor (n, number of free radicals trapped by one mole of antioxidant moiety) for the AIBN system declined in the order BMP (2.0), 2-hydroxyestradiol (2.0) > DES (1.3) > BPA (1.2) > estrone (0.9), whereas that for the BPO system declined in the order BMP (2.0) > DES (1.9), BPA (1.9) > estrone (1.3) > 2-hydroxyestradiol (0.7). The inhibition rate constant (k(inh) x 10(-3) M(-1)s(-1)) for the AIBN system declined in the order estrone (2.2) > BPA (2.0) > DES (1.9) > 2-hydroxyestradiol (1.2) > BMP (1.1), whereas that for the BPO system declined in the order 2-hydroxyestradiol (3.2) > estrone (1.4) > DES (1.2) > BPA (1.0) > BMP (0.9). The radical-scavenging activity for bioactive compounds such as estrogens should be evaluated using these two methods (the n and kinh) to elucidate the mechanism of a particular reaction. The great difference of the n and kinh for estrogens between the AIBN and BPO system suggested that their oxidation process is complex.