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.