The article discusses an evaluation of the effect of percent outdoor air supplied and occupation level on the particle size distributions and mass concentrations for a typical office building. (NOTE: As attention has become focused on indoor air pollution control, it has become important to obtain basic information on the effects of heating, ventilating, and air-conditioning system parameters on office aerosols. In addition, it is important to know the particle size distributions in a typical office environment.) The outdoor, return, and supply air streams, as well as hallway air, were sampled using measuring equipment covering particle diameters from < 0.1 to > 10.0 micrometers. The mass concentrations, when the building was occupied, increased by a factor of about 2 when all return air was used over all outdoor air. The concentrations when unoccupied using no outdoor air were as low or lower than were those when the building was occupied using all outdoor air. All of the occupied concentrations were < 200 micrometers/cu m. As expected, the outdoor air was cleaner than the other streams. The next lowest concentrations were for supply air, then return air, with hallway air showing the highest concentrations. The normalized number distributions had a single mode consistently near 0.13 micrometer; the volumetric distributions peaked at 0.3 micrometer.