To determine the relative ranking of antioxidative potential of various steroids the effect of 14 steroid compounds on the fluorescence of phycoerythrin was monitored over time following the addition of a peroxy radical generator 2,2'-azo-bis (2-amidino-propane) dihydrochloride. The rate of decay of fluorescence in the presence of a 200 nM of 17 beta-estradiol, 17 alpha-estradiol and estriol expressed as percentages of the rate of decay in the absence of these compounds (control curve), were 74.1 +/- 6.3, 84.0 +/- 5.42 and 64.2 +/- 2.53%, respectively (P < 0.005). Cortisone and corticosterone appeared to have very mild pro-oxidant properties. Other steroids tested such as estrone, testosterone, progesterone, androstenedione, dehydroepiandrosterone, cortisol, tetrahydrocortisone, deoxycorticosterone and aldosterone had no significant antioxidant properties. It is concluded that estrogens especially estriol and 17 beta-estradiol are naturally occurring antioxidants.