Several recent studies have reported elevated levels of particulate matter inside school bus passenger compartments. These studies have gained national attention since children are particularly susceptible to adverse health affects from diesel particulate matter. The results of studies conducted to date have varied from study to study and controversy still exists as to the source of the elevated PM. The objectives of the present study are to 1) evaluate the relative contribution of emissions from both the crankcase and the tailpipe toward in-cabin levels of fine and ultrafine particles; and (2) determine whether technologically feasible reductions in tailpipe emissions would significantly reduce the health risks associated with exposure of children to particles in the cabin of a standard school bus. To satisfy these objectives, mobile testing will be conducted with a school bus powered by an International DT466E engine on an outdoor test track at the Aberdeen Test Center in Aberdeen, MD. Tests will follow a drive cycle developed using GPS data from real school bus routes. Particulate matter concentrations will be measured using three Thermo Electron DataRAM 4 units, three TSI P-Trak ultrafine particle counters and the Sensors PM-300 tailpipe particulate analyzer. Gaseous emissions (CO, CO2, HC, NOx) as well as pertinent engine parameters such as engine speed, fuel flow rate, engine oil temperature, and percent engine load will be measured using the Sensors SEMTECH-D tailpipe emissions analyzer, which can be interfaced with the vehicle electronic control module. Tests will be conducted with the school bus fitted with a closed crankcase filtration system, a diesel particulate filter and a diesel oxidation catalyst.