Aqueous suspensions of virulent Legionella pneumophila grown on solid medium retained virulence and aerosol survival characteristics for several months. Significant numbers of viable organisms were recovered from aerosols held at various relative humidities (r.h.) for up to 2 h. The organisms survived best at 65% r.h. and were least stable at 55% r.h. Exponential phase broth-grown organisms survived poorly in aerosols in comparison with stationary phase broth cultures or organisms grown on solid medium, suggesting that the metabolic status of Legionella pneumophila organisms may be an important factor affecting their ability to survive in aerosols and cause respiratory disease.