To test the possible favoring effect of inhaled inert dust on the spontaneous microflora of respiratory surfaces of rodents, groups of mice, hamsters, and rats were exposed to graphite (1 mg/cu m) and calcium sulfate (1 mg/cu m) for 12 hours daily, for periods up to four months. A limited number of hamsters have been exposed to high concentrations of graphite dust (10 mg/cu m) continuously (23 hr/day) up to 32 consecutive days. In general, particulate aerosols do not appear to enhance colonization of bacteria or stem bronchi and air spaces. However, a fraction of the hamsters exposed to graphite, 10 mg, developed Mycoplasma pulmonis. These observations offer an encouraging hypothesis in support of the possible effect upon Mycoplasma infection following exposure to particulate pollutants.