Aims: To study the effects of the delivery vehicle for Lactobacillus acidophilus on the human faecal microbiota. Our hypotheses were that (i) the delivery vehicle would influence faecal lactobacilli numbers and (ii) consumption of Lact. acidophilus would influence the populations of Bifidobacterium and hydrogen sulphide-producing bacteria. Methods and Results: Ten subjects each received Lact. acidophilus with skim milk or water. Lactobacillus, Bifidobacterium and hydrogen sulphide-producing bacterial populations were analysed before, during and after each treatment. Regardless of the vehicle, faecal lactobacilli populations changed during treatment. Bifidobacteria and the hydrogen sulphide-producing bacteria underwent no statistically significant population changes. Intra- and intersubject variability was observed. Conclusions: The vehicle in which Lact. acidophilus was delivered did not influence faecal lactobacilli numbers. Consumption of Lact. acidophilus did not influence the populations of Bifidobacterium and hydrogen sulphide-producing bacteria. The lactobacilli populations of subjects were variable. The fed lactobacilli did not appear to colonize the gastrointestinal tract. Significance and Impact of the Study: We provide evidence that (i) there was no collective advantage to using skim milk as a delivery vehicle vs water; (ii) exogenous Lact. acidophilus did not affect endogenous bifidobacteria or hydrogen sulphide-producing bacteria; (iii) data should be carefully examined before pooling for analysis and (iv) continuous feeding was required to maintain an elevated lactobacilli population.