The peroperative flora of 499 patients undergoing cataract extraction was studied with local bacterial cultures taken at the beginning and end of surgery and compared with the preoperative flora examined previously (Fahmy et al. 1975 b) on admission the day prior to surgery. The local application of a single dose of oxytetracycline - polymyxin B, approximately 18 hours before surgery, significantly reduced the incidence of bacteria at the time of surgery. However, 92% of the conjunctivas examined immediately before operation proved to harbour one or more kinds of microorganisms. Futhermore, 61% of the wound sites were found to be contaminated with bacteria at the conclusion of surgery. The reasons are discussed. The origin of Staphylococcus aureus isolated peroperatively from the conjunctiva and wound site was studied. The great majority of strains could be traced to the patient's own conjunctiva preoperatively. In a few cases S. aureus was traced to the patient's own nose, skin of face or to the surgeon's nose. The air of the wards and operating theatre as well as the hands and gloves of surgeons and assistant nurses apparently did not play any role as a source of S. aureus infection.