A prospective bacteriologic investigation was made in 43 consecutive patients (mean age 63 years) operated on for acute cholecystitis. Gallbladder bile and wall were cultured, using four methods and with special attention to optimal anaerobic technique. Cultures were positive in 72% of the patients, yielding a wide variety of species (21 species among 48 isolates). Anaerobes constituted 23% of the isolates. Cultures from gallbladder bile and from gallbladder wall gave almost identical results, as did sampling at the beginning and at the end of cholecystectomy. Bactibilia was found in all patients operated on within 48 hours after the onset of symptoms. Bactibilia and postoperative septic complications showed statistically significant correlation with high patient age. Bactibilia and gallbladder gangrene were significantly correlated with preoperative temperature greater than 38.5 degrees C. There was coincidence of strains isolated from local wound sepsis and from peroperatively sampled gallbladder bile. Adequate preoperative or peroperative antibiotic therapy according to susceptibility testing was associated with significantly reduced rate of postoperative septic complications. The study indicates that bacteria are present early in the course of acute cholecystitis and that they are causally important for postoperative morbidity and mortality.