Electronic thermometers used in many hospitals require insertion into the mouth of a temperature-sensing probe covered by a pre-packaged probe cover. Handling procedures used by hospital personnel can lead to inadvertent manual contact with the probe cover. We attempted to determine the rate of non-sterility of these probe covers under normal hospital conditions and the extent to which pathogens could be detected on these and other components of the thermometer. Probe covers were removed before entry into the patient's mouth and aseptically inoculated into fluid thioglycollate medium. Subcultures were made to appropriate differential media. Over a 13-week period, 180 covers were cultured on two private hospital services. Forty-three percent were found to be non-sterile; control covers from central supply were only 6% non-sterile. Four percent of the probe covers harbored potential pathogens: coagulase positive Staphylococcus aureus, Streptococcus pyogenes, Enterobacter, and Bacteroides. No attempt was made to correlate these findings with infection. These results indicated a potential infection hazard for high-risk patient groups and a need by hospital personnel to consider this as a possible source of contamination [Infect Control 1981; 2(4):315-6.]