In this pilot-scale, double-blind, placebo-controlled trial, 30 patients with Helicobacter pylori infection were randomised into three groups prior to their 7 days eradication therapy, to study the effects of probiotic supplement comprising Lactobacillus acidophilus and Bifidobacterium bifidum on the intestinal microflora in response to antibiotic therapy. Group I received the placebo product from day 1 to day 15, Group II received placebo from day 1 to day 7 and probiotics from day 8 to day 15 and Group III received probiotics from day 1 to day 15. Patients provided stool samples for analysis on days 1, 7, 12, 17 and 27. For patients in Groups I and II, significant increases in the facultative anaerobe component of the microflora occurred between days 1 and 7. In Group I, the numbers remained elevated to day 27 but in Group II, the numbers decreased significantly between days 7 and 27 back to the starting levels. In Group III, the facultative anaerobe population remained stable throughout. The total anaerobe numbers increased significantly at day 27 than at day 1 for Group I, were unchanged throughout for Group II and decreased significantly for the patients in Group III between days 1 and 7 before reverting to the starting levels by day 27. From these results, it can be seen that probiotic supplementation modulates the response of the intestinal microflora to the effects of antibiotic therapy.