We previously described the effects of intake of dairy products on plasmid dissemination in the digestive tract of gnotobiotic mice associated with human faecal flora (HFF) and found that yoghurt, heat-treated yoghurt (HTY) and milk reduced population levels of transconjugants compared with findings in mice fed a standard mouse diet. In the case of lactose intake, transconjugants were not detected. The aim of the present study was to assess the possible interrelationships between these observations and other variables (bacterial ecology, pH, moisture, enzyme activities, short-chain fatty acid (SCFA) contents, lactic acid contents). Much of the interest of the present comparison lies in the fact that the animals were homogeneous in terms of age, gender, food and intestinal microflora, owing to the gnotobiotic mouse model maintained in sterile isolators. We observed no variation in SCFA and lactic acid contents or in the population levels of strictly anaerobic strains of Bacteroides and Bifidobacterium, and of the facultative anaerobic recipient Escherichia coli PG1 strain. The main modifications were the reduction of population levels of transconjugants in mice receiving yoghurt, HTY and milk, and concomitantly an increase of beta-galactosidase and a decrease of beta-glucosidase activities, compared with control mice fed a standard diet. Total inhibition of plasmid transfer was observed in HFF mice consuming lactose, and concomitantly the two enzyme activities (beta-glucosidase and beta-galactosidase) were increased, compared with the findings in control mice fed a standard diet. In axenic mice consuming lactose, plasmid transfer occurred, beta-galactosidase was not detected and beta-glucosidase was decreased. It is therefore proposed that these two enzyme activities influence plasmid transfer and persistence of transconjugants in the digestive tract of HFF associated mice. When both activities were increased there was a total inhibition of plasmid transfer (case of lactose intake). When beta-galactosidase increased and beta-glucosidase decreased (case of yoghurt, HTY and milk), plasmid transfer occurred at a lower efficiency than in the control group, resulting in lower population levels of transconjugants.