Studies suggest that asymptomatic colonization with Clostridium difficile (CD) decreases the risk of CD-associated disease (CDAD) in humans. A hamster model was used to test the efficacy of colonization with 3 nontoxigenic CD strains for preventing CDAD after exposure to toxigenic CD. Groups of 10 hamsters were given 10(6) nontoxigenic CD spores 2 days after receiving a single dose of clindamycin. Five days later, the hamsters were given 100 spores of 1 of 3 toxigenic CD strains previously shown to cause mortality within 48 h. Each nontoxigenic strain prevented disease in 87%-97% of hamsters that were challenged with toxigenic strains. Failure to prevent CDAD was associated with failure of colonization with nontoxigenic CD. Colonization with nontoxigenic CD strains is highly effective in preventing CDAD in hamsters challenged with toxigenic CD strains, which suggests that use of a probiotic strategy for CDAD prevention in humans receiving antibiotics might be beneficial.