A rifampicin-resistant Lactobacillus salivarius was isolated from chicken caeca and administered orally to newly hatched broiler chickens. The resistance to rifampicin enabled to differentiate the administered organism from indigenous strains. After inoculation, rifampicin-resistant lactobacilli dominated among lactobacilli in the crop and caeca of inoculated chickens. In two different experiments the inoculation by L. salivarius lowered count of enterococci in the crop in the whole experimental period (6 and 3 days in the first and the second trial, respectively). Counts o f coliform bacteria in the crop were lowered to a smaller extent, in the first day in both trials only. Effects of L. salivarius administration on caecal counts other than lactobacilli were generally small. Four methods of L. salivarius administration were compared: live cells per os, lyophilized per os, live cells via drinking water and lyophilized cells added to the feed mixture. Rifampicin-resistant lactobacilli became established in the crop and caeca using any method of inoculation tested. Provision of lactobacilli decreased the pH of the chymus, especially in the first day of experiment.