Sixteen healthy volunteers, 23 patients with peptic disease, and 38 patients with achlorhydria were investigated with bacterial cultures from gastric juice and a hydrogen (H2) breath test after a standard meal. In acid-secreting subjects upper respiratory tract bacteria were found in 6/39. In every single achlorhydric patient gastric bacterial concentrations were above log 10 5.7/ml. Fecal flora was found in 22/38. The patient with fecal organisms were significantly older than those without (p less than 0.05). The H2 breath test revealed low postprandial carbohydrate fermentation in acid secretors and in achlorhydrics with no fecal flora. The lowest H2 production was found 90 and 120 min after the meal. In achlorhydrics with fecal organisms the mean H2 concentration at these times of measurement was elevated (p less than 0.05). Assuming that a mean H2 concentration at 90 and 120 min above 19 ppm (mean in healthy volunteers at 90 and 120 min + 2 SD) is pathological, the association of fecal bacteria in gastric juice and a "positive" H2 breath test is highly significant (P less than 0.001). Fecal bacteria in the gastric juice would indicate small-intestinal fermentation with a probability of 81%. A gastric culture negative for fecal organisms would predict a "negative" H2 breath test with a probability of 91%.