Objective The supernatant of Lactobacillus johnsonii La1 culture was shown to be bactericidal and to have a partial, acid-independent suppressive effect on Helicobacter pylori in humans. The aim of the present study was to investigate the effect of L. johnsonii La1-acidified milk (LC-1) on H. pylori infection, Design and methods Fifty-three volunteers infected with H. pylori as determined by positive C-13-urea breath test and positive serology were randomized to receive either LC-1 or a placebo 180 mi twice a day for 3 weeks. All subjects also received clarithromycin 500 mg bid during the last two weeks of acidified milk therapy. Oesophagogastroduodenoscopy and biopsies were performed at inclusion and repeated 4-8 weeks after the end of the treatment, H. pylori infection was confirmed by urease test and histology, H. pylori density and inflammation were scored using a modified Sydney classification. Results LC-1 ingestion induced a decrease in H. pylori density in the antrum (P = 0.02) and the corpus (P = 0.04). LC-1 also reduced inflammation and gastritis activity in the antrum (P = 0.02 and P = 0.01, respectively) and of activity in the corpus (P = 0.02), Clarithromycin eradicated H. pylori in 26% of the subjects; LC-I did not improve the antibiotic effect. Conclusion These results suggest that H. pylori infection and gastritis can be down-regulated by LC-I. Eur J Gastroenterol Hepatol 13:25-29 (C) 2001 Lippincott Williams & Wilkins.