We characterized a tobramycin aerosol generated by five nebulizers: Micron One, Pulmosonic, Pulmo-Aide, DeVilbiss Model 65, and UltraNeb 100 by particle size and drug concentration. The Micron One nebulizer did not produce a recoverable aerosol, while the Pulmosonic had a minimal output; therefore three machines were examined for their ability to deliver tobramycin to the lower respiratory tract of patients with cystic fibrosis (CF). The DeVilbiss 65 had the greatest output: with air as the carrier gas it produced an aerosol with > 60% of the particles having a mean mass aerodynamic diameter (MMAD) of > 5.5 microns. Using helox shifted the MMAD so that > 65% of the particles were < 5.5 microns. Increasing the power in the DeVilbiss 65 increased the output of particles > 9.2 microns, without a change in the particles < 3.3 microns. With air as the carrier gas the Pulmo-Aide and the UltraNeb 100 produced an aerosol with > 60% particles, < 3.3 microns MMAD. Using helox the UltraNeb 100 increased the amount of aerosol with a < 3.3 microns MMAD to 98%. Tobramycin delivery to the lower respiratory tract with the Pulmo-Aide and UltraNeb 100 was compared using air or helox by measuring sputum drug concentration. Pulmo-Aide failed to produce detectable tobramycin in sputum in 2 out of 9 patients with CF. With the UltraNeb 100, all patients had measurable sputum tobramycin immediately after administration (range, 16.2-3385 micrograms/g), but no statistically significant difference was found when using either compressed air, helox, or ambient air.(ABSTRACT TRUNCATED AT 250 WORDS)