Being a human isolate and bacterocin producer with wide antimicrobial spectrum, Lactobacillus acidophilus LF221 fulfills preliminary conditions as a potential probiotic strain. To investigate the LF221 strain with respect to its persistence in complex ecosystems, viable LF221 cells were orally administered to laboratory animals (mice and piglets) and added to cheese milk. During the 10-day feeding of mice with LF221 viable cells, 0.9% of lactobacilli isolated from faeces were identical with LF221 strain. Within the piglets' feeding trial, 920 lactobacilli colonies randomly selected from faecal samples were screened for the presence of LF221-like colonies. During the six times of sampling of piglets' faeces, LF221 colonies were always detected. Even the final sampling 10 days after the last LF221 cell suspension was dosed into piglets revealed the presence of LF221 strain at a concentration of about 3 x 10(8) cells/g of faeces. LF221 intake did not significantly influence the total count of lactobacilli in faecal samples and it had no negative health effects on animals. Analyses of cheese demonstrated that the LF221 strain maintained a high viability during ripening and that it could be detected for at least 6 weeks at a level of about 6.8 x 10(6) cells/g of cheese. According to these results, semihard cheese represents a suitable medium for introducing a potential probiotic LF221 into a diet.