Abstract Background Use of synbiotic preparations as dietary supplement is believed to be a valid approach to restore and maintain colonic microflora. However, only few papers have been published on the assessment of these food supplements and none of them have used molecular biology techniques to evaluate the effects of the probiotic components. Methods Twelve healthy volunteers were recruited. Faecal samples were taken before and at various time points during the administration period and at day 3 in the post-treatment period. Stool culture were performed and amplified ribosomal DNA restriction analysis was used to detect L. paracasei , the major bacterial component of the synbiotic products. Results An increase of at least 1 log of L. paracasei -like bacteria was observed in all subjects. An increase of as much as 3 log was seen in subjects who had a low number of L. paracasei -like lactobacilli at the baseline. The counts of L. paracasei -like lactobacilli were found to persist for at least 3 days after discontinuation of intake in healthy volunteers in 7 subjects. Genetic analysis showed that the maiority of vancomicin insensitive lactobacilli were real L. paracasei , as the strains administered with the tested product. Conclusion This study has shown that the strains of L paracasei administered with a synbiotic dietary supplement are able to survive through the gastrointestinal tract and to persist for at least a few days. It was also shown the efficacy of a synbiotic preparation to positively affect the microflora of healthy volunteers.