Background: Several bacterial strains, mainly those belonging to the genera Lactobacillus, Enterococcus, Streptococcus and Bi dobacterium, are currently used as probiotics. Objective: The aim of this study was to investigate the in uence of probiotics on ve biochemical microbial-related functions in vitro and in vivo, in gnotobiotic mice, by applying the G AC:M AC (germ-free animal characteristic:micro ora-associated characteristic) concept. Design: Sixteen probiotics were monoinoculated in vitro and:or monoassociated to germ-free mice to investigate the following biochemical parameters in large intestinal samples: inactivation of tryptic activity, degradation of b-aspartylglycine and of mucin, conversion of bilirubin to urobilinogen and b-glucuronidase activity. Results: Lactobacillus reuteri 2010, L. rhamnosus strain 271, L. rhamnosus ATCC7469 and L. rhamnosus G G ATCC53103 expressed b-glucuronidase activity. The other parameters tested were not in uenced by any of the probiotics. Conclusions: Bacterial b-glucuronidases have an important intestinal function. As many microbial -host and microbe-diet interactions occur in the gastrointestinal tract of mammals, microbial-related functions should be studied in greater detail in probiotics.