Nosocomial transmission of respiratory pathogens is a possible complication of lung function testing. The use of a filter, placed between the patient and the spirometer equipment may be one way of preventing such nosocomial spread. This paper reports the laboratory evaluation of the efficiency of such a filter at removing bacteria from exhaled breath. Volunteers exhaled 100 normal breaths or four forced exhalations through a filter on to a blood agar plate. Bacterial counts on the agar plate were compared with recoverable bacterial counts from the filter. Total challenge from the forced exhalations ranged from 161-84,200 colony forming units. The calculated efficiency of 10 volunteers was 99.9%. For the normal breaths the challenge ranged from 0-262,000 colony forming units. There was no growth on any of the blood-agar plates. These filters appear to be highly efficient at removing exhaled bacteria.