The occurrence of bactibilia was studied in a series of 515 patients operated on at different stages of acute cholecystitis (AC). The gallbladder bile cultures showed highest incidence of bactibilia in patients operated on within 24 hours of the onset of AC (63%). Significantly lower incidence (31%, p less than 0.001) was found only in the group receiving delayed surgery greater than or equal to 11 days later. The use of prophylactic antibiotics had only minor, if any, effect on the occurrence of positive bile cultures. The frequency of different bacterial strains did not differ significantly at various stages of AC. It is concluded that the gallbladder bile becomes infected so early in the course of AC that the risk of increased infection complication rate after surgery is present even from the very beginning. The possible benefit of prophylactic antibiotics in early surgery for AC remains open on the basis of the present study.