The efficacy of autoclaving high-speed dental handpieces was tested in two types of downward displacement steam sterilizers (instrument autoclaves), commonly used in the dentist's office. Eight series of experiments were performed with various sterilization schedules. Lubrication oils with or without antimicrobial agent were used in four of the series. Each handpiece was inoculated with 1 ml of a suspension containing equal amounts of saliva and spores ofBacillus stearothermophilus(approx. 106spores/ml). Neither sterilization at 120—124°C for 20 min nor at 134—136°C for 10 min resulted in complete inactivation of the spores in series in which the instruments were tested without oil or with oil containing no antimicrobial agent. However, when the handpieces were lubricated with oil containing isopropanol and formaldehyde and the schedule 134—136°C/10 min was used, no growth was observed. The results indicate that instrument autoclaves with built-in programs of 120—124°C/20 min and 134—136°C/10 min could have insufficient capacity to sterilize lubricated or unlubricated dental handpieces. The use of oils containing an antimicrobial agent can overcome this problem.