Describes the microbioligic surveillance of the central air handling systems in a hospital, prompted by an increase in the rates of nosocomial infection caused by filamentous fungi in immunocompromised patients. Heavy contamination by Penicillium was found in ventilation ducts and terminal unit lined with fiberglass in the operating room air handling system, as well as Aspergillus species. The contamination was corrected using filter replacement and decontamination with aerosolized chlorine solution.