Aerosol sampling of ultrasonic nebulizers (USN) during routine use revealed significant contamination by gram negative bacteria in each instance. Exposure of a sterile USN machine to a patient harboring Serratia in sputum resulted in heavy growth of Serratia in all parts of the USN after 48 hours. Acetic acid in a concentration of 0.25% was not effective in cleaning heavily contaminated machines, whereas 2% acetic acid for 30 minutes rendered the USN bacteria free. The USN are hazardous insofar as they contain water reservoirs in which certain gram negative organisms multiply rapidly. Hospitals must have a regular system of adequate cleaning and regular surveillance, preferably of aerosols produced, in order to prevent infecting patients requiring inhalation therapy.