The gastric microflora of 21 cases receiving histamine H2-receptor antagonists (H2-blocker) were studied. Patients not given H2-blocker were also examined for gastric microflora. The gastric pH of patients who were receiving H2-blocker ranged from 1.91 to 6.51 (mean, 4.32). The gastric pH of over 5.00 was seen in 10 cases. Aerobic bacteria were isolated from 20 cases (95%). Aerobic bacterial counts per ml of gastric aspirates correlated positively with the pH of those aspirates but was little correlation between the pH of the gastric aspirates and the number of anaerobic bacteria of the samples, which were positive in 7 cases (33%). Candida and Staphylococcus were predominant isolates with positive rates of 82% and 41%, respectively. While Fusobacterium was isolated from 33% of the samples, none of the Bacteroides fragilis group organisms were isolated. It was also observed in the control group that aerobic and anaerobic bacteria were isolated from the gastric aspirates with high pH. These results suggest that in patients receiving H2-blocker anaerobes as well as aerobes may have the possibility to become pathogen for aspirate pneumonia and so on because of the overgrowth of anaerobic bacteria in the gastric fluid.