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The probiotic potential of 47 selected strains of Lactobacillus spp. was investigated. The strains were examined for resistance to pH 2.5 and 0.3% oxgall, adhesion to Caco-2 cells, and antimicrobial activities against enteric pathogenic bacteria in model systems. From the results obtained in vitro, five strains, Lactobacillus rhamnosus 19070-2, L. reuteri DSM 12246, L. rhamnosus LGG, L. delbrueckii subsp. lactis CHCC 2329, and L. casei subsp. alactus CHCC 3137, were selected for in vivo studies. The daily consumption by 12 healthy volunteers of two doses of 10(10) freeze-dried bacteria of the selected strains for 18 days was followed by a washout period of 17 days. Fecal samples were taken at days 0 and 18 and during the washout period at days 5 and 11. Lactobacillus isolates were initially identified by API 50CHL and internal transcribed spacer PCR, and their identities were confirmed by restriction enzyme analysis in combination with pulsed-field gel electrophoresis. Among the tested strains, L. rhamnosus 19070-2, L. reuteri DSM 12246, and L. rhamnosus LGG were identified most frequently in fecal samples; they were found in 10, 8, and 7 of the 12 samples tested during the intervention period, respectively, whereas reisolations were less frequent in the washout period. The bacteria were reisolated in concentrations from 10(5) to 10(8) cells/g of feces. Survival and reisolation of the bacteria in vivo appeared to be linked to pH tolerance, adhesion, and antimicrobial properties in vitro.