The aim of this study was to evaluate the effect of Lactobacillus rhamnosus IMC 501 (R) and Lactobacillus paracasei IMC 502 (R) on oxidative stress in athletes during a four-week period of intense physical activity. Two groups of twelve subjects each were selected for this analysis. The first group consumed a daily dose of a mixture of the two probiotic strains (1:1 L. rhamnosus IMC 501 (R) and L. paracasei IMC 502 (R); similar to 10(9) cells/day) for 4 weeks. The second group (control) did not consume any supplements during the 4 weeks. Blood samples collected immediately before and after the supplementation were analyzed, and plasma levels of reactive oxygen metabolites and biological antioxidant potential were determined. Faeces were also collected and analyzed before and at the end of the probiotic supplementation. Antioxidative activity and oxidative stress resistance of the two strains were determined in vitro. Results demonstrated that intense physical activity induced oxidative stress and that probiotic supplementation increased plasma antioxidant levels, thus neutralizing reactive oxygen species. The two strains, L. rhamnosus IMC 501 (R) and L. paracasei IMC 502 (R), exert strong antioxidant activity. Athletes and all those exposed to oxidative stress may benefit from the ability of these probiotics to increase antioxidant levels and neutralize the effects of reactive oxygen species.