The Aerotest sampler (Olympic Medical Corp., Seattle, Wash.) is designed to detect bacterial contamination of respirator-generated aerosols in a simple one-step process. The sampler was evaluated by controlled contamination of Bennett AP-5, PR-2, and MA-1 respirators with Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter. Bacteria present as viable 1.4 to 3.5-mum particles in each of 77 aerosols were quantitated by using the Andersen viable-particle sampler, and the results were compared with those of colonies obtained from 10-s or 15-cycle Aerotest samples. All aerosols containing counts greater than expected in hospital air were detected by the Aerotest method. Thirteen of 14 aerosols containing less than 20 bacteria/0.028 m3 showed fewer than 5 colonies, whereas all aerosols containing greater than 1,000 bacteria/0.028 m3 showed at least 40 colonies. Aerosols with intermediate counts fell in between. Under the conditions described here, the Aerotest sampler was able to discriminate between the low levels of bacteria commonly found in hospital air and the bacterial contamination associated with nosocomial pneumonia. The Aerotest sampler provides a practical method for surveillance sampling of respirator nebulizers.