The aim of this study was to evaluate the effect of Lactobacillus rhamnosus IMC 
501 and Lactobacillus paracasei IMC 502 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 and L. paracasei 
IMC 502; ~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(®) and L. paracasei IMC 
502(®), 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.