Heat and Moisture Exchangers are used increasingly as filters to prevent the contamination of breathing apparatus and to limit cross-infection. A series of laboratory tests has been developed to evaluate the microbial filtration and air flow resistance properties of these devices. The tests were designed to simulate the clinical situation and therefore evaluated devices in both dry and wet conditions. The devices tested (Engstrom Edith, Pall BB50T, Dar Hygrobac, Intersurgical Filtatherm and Intersurgical Filtaguard) were each representative of a particular type of construction. The simple hygroscopic device (Engstrom Edith) showed poor airborne and liquid-borne filtration efficiency, but its resistance to air flow remained low in all conditions. The composite devices (Dar Hygrobac, Intersurgical Filtatherm and Intersurgical Filtaguard), which all possessed relatively large pores, performed well in terms of dry airborne filtration efficiency, but showed substantial increases in air flow resistance and poor filtration efficiency in the presence of liquid. The pleated membrane filter (Pall BB50T), which possessed small pores, showed good airborne filtration efficiency and prevented the passage of liquid. The latter property enabled this device to prevent the passage of liquid-borne contamination and to maintain a low resistance to air flow in wet conditions. It would appear that in terms of contamination control and air flow resistance the pleated membrane filter provides a wider margin of safety than either the hygroscopic or composite devices.