The paper gives results of a study of particle concentrations and size distributions in a typical office building, as a function of makeup air percentages and sources. Filters of the type in use were obtained and tested for particle size dependent efficiencies. Activity and sources in the building were determined, and settings for the dampers controlling makeup air were either fully open or shut. Testing was done during occupied and relatively unoccupied periods. In-room, return, outside, and just-filtered combined air streams were sampled over a particle size range from 0.01 to 10 micrometers. Particle size distributions, mass concentrations, temperature, and humidity were determined.