DeVilbiss No. 40 hand-held nebulizers are widely used for quantifying airway responsiveness in large populations using pharmacological agents. We examined the aerosol characteristics of five nebulizers. Within each device, the aerosol output and droplet size were reasonably stable over a wide range of bulb pressures, although there were considerable differences in output characteristics between nebulizers. The droplet size was very large compared to conventional aerosol delivery systems, with a mass median diameter greater than 10 microns for three of the five devices. Between 28-50% of the output was in particles sufficiently small for airway deposition (less than 6.2 microns). A more vigorous compression of the bulb caused a small increase output and a reduction in droplet size, resulting in a much bigger variation in the output of the respirable aerosol (less than 6.2 microns) with changes in bulb pressure. The loss due to evaporation was about 3.5%, causing a similar rise in the osmolality of the nebulizer solution. In view of the variable nebulizer output and the marked between-operator variation in bulb pressure, the characteristics of individual DeVilbiss No. 40 nebulizers should be evaluated by individual operators before use in clinical practice or research.