In a study of 28 ventilated patients in the ICU, cimetidine was ineffective in maintaining gastric pH above 4. Quantitative and qualitative bacteriological examination of daily gastric aspirates showed that when the pH was above 4, there was rapid colonization with high counts of organisms, predominantly coliforms. Progressive colonization by yeasts, independent of pH, was noted in nearly one-half of the patients. Gastric colonization has possible implications in terms of crossinfection or development of aspiration pneumonia. As these are seriously ill patients with compromised gastrointestinal (GI) barriers and decreased immunity, the large numbers of bacteria or their endotoxins may contribute to the high incidence of septicemia.