The ability of pigs to neutralize the reactive oxygen species (ROS) plays a key role in their welfare. Two micromethods were developed to evaluate the oxidative stress and the anti-oxidative response in sera. We tested these methods for linearity, repeatability and reproducibility. A hydroxyl radicals (HR) test, based on the Fenton reaction, showed a linearity between 0.26 and 4.5 mM H2O2 (r = 0.997), with a coefficient of repeatability (CVr) of 1.9 and a coefficient of reproducibility (CVR) of 2.9. An anti-oxidant power (AOP) test, based on the capability of sera to neutralize a titred hypochlorous acid solution (HClO), showed a linearity between 55 and 880 microM HClO neutralized (r = 0.984), with CVr = 3.1 and CVR = 4.9. We assessed the applicability of such tests in field conditions on three different farms. Farm A recorded a positive anamnesis of Mulberry heart disease. Dietary Vitamin E supplementation was given at 50 p.p.m. in farms A and B and at 150 p.p.m. in farm C, respectively. The group from farm A showed HR sera levels higher than those of farm B and farm C (farm A, mean = 31.61 +/- 2.87 mM H2O2, n = 10; farm B, 28.53 +/- 2.30, n = 10; and farm C, 25.63 +/- 1.03, n = 10; P < 0.01). The AOP test was not able to discriminate between farms. The AOP levels found (mean = 472.50 microM HClO neutralized: n = 30) probably represent the maximum response of the organism to compensate the oxidative stress. HR and AOP are stable in sera stored at +4 degrees C. We discuss the application of such methods for the assessment of farmed animals' welfare.