Production and manufacturing methods and the food carrier may influence the properties of probiotic strains, and have an impact on the outcome of clinical intervention studies. The aim of the present study was to establish whether the properties of a specific probiotic strain,Lactobacillus rhamnosusGG, may differ depending on the product and source of the strain. In total, fifteen differentL. rhamnosusisolates, among them fourteen labelled asL.rhamnosusGG, were isolated from specific probiotic products. The micro-organisms were phenotypically and genotypically characterised. Their adhesion properties were compared using the human intestinal mucus model, and the ability of the isolates to influence model pathogen adhesion to human colonic mucus was assessed. AllL. rhamnosusisolates used were confirmed as members of the speciesL. rhamnosus. Except the reference strain OL, allL.rhamnosusisolates showed randomly amplified polymorphic DNA, enterobacterial repetitive intergenic consensus and pulsed-field gel electrophoresis profiles identical to that ofL. rhamnosusGG (ATCC 53103). AllL.rhamnosusisolates showed similar tolerance to acid and were able to bind to human colonic mucus. However, pathogen exclusion by inhibition and competition varied significantly among the differentL. rhamnosusisolates and pathogens tested. The results suggest that different sources of the same probiotic may have significantly altered strain properties. This should be considered inin vivostudies on human subjects and also for quality control of probiotic products.