Mycoplasma pneumoniae, a common human respiratory pathogen, has been studied experimentally for years using intranasal inoculation of the golden Sytrian hamster. Because of recent evidence outlining the role in pulmonary immune development of particle size and depth of mycoplasma deposition in the hamster lung, the authors developed an aerosol chamber for the reproducible aerosolization of radiolabeled M. pneumoniae. Organisms were labeled to high specific activity by the incorporation of 3H-oleic acid and aerosolized under air-flow and humidity conditions creating a mean particle diameter of 2.0 micrometers. Under these conditions, viable mycoplasmas were reproducibly and evenly distributed to all major lobes of the lung. Examination of radioactive clearance and organism viability within the lung during the first 48 hr after aerosolization have suggested a minimal role for macrophage mycoplasmacidal activity and a more prominent role for ciliary clearance. Data from aerosol infections of hamsters with radio-labeled M. pneumoniae should provide a unique opportunity to examine in a highly controlled manner the effects of air pollutants on the initial stages of infection as well as effects on the development of pulmonary immunity and histologic alterations.